- Please keep these annotations SPOILER-FREE by not revealing information from later pages in the novel.
44/39 - X
"...the Xs of the grating in the middle of the mall." An X is formed by sticking two Vs together (one upside down).
Stencil was born the year Queen Victoria of England died. The name Victoria may also refer to a character later in the novel.
Kind of opposite of a yo-yo's movements.
But just as goalless, perhaps?
In music terminology, the mixolydian mode is a major scale with a flatted, aka minor or (appropriate to "the laziest living creature in New York") "lazy" seventh degree. The mixolydian is also the fifth mode-in the key of C major, the fifth note of the major scale is G, so if you play a scale from G to G, but keep the key signature of C major, you have Mixolydian(all white keys on the piano). It is labeled with the Roman Numeral V in music theory, and usually resolves to the tonic key, or I(C in the example). The movement from I to _ (often IV; ii in jazz)to V and back to I is, simply stated, the basis of Western music harmony. Schoenberg later dismantles this with the creation of serialism, where all notes are treated democratically.
Fergus is man-strength or virility. Fergus Mixolydian could also be a reference to Maynard Ferguson, jazz trumpet player and leader of the Birdland Dream Band in 1956 who lived for some time with Timothy Leary. Pynchon references Birdland in his 1959 short story "The Small Rain."
Arnold Schoenberg devised serialism, a new approach to organizing musical notes that doesn't rely on the diatonic scale (with its whole and half steps giving certain notes prominance over other notes and creating tonal polarization). According to strict serialism, all twelve notes of the chromatic scale are used, arranged in rows, and each note in the row must be played in order. Thus, all the notes have equal weight, and by analogy serialism can be seen as entropic in that it moves from the asymmetry of tonal polarization towards symmetry and equality of notes. As Gustav Schlabone says in Gravity's Rainbow about another German who pushed the envelope, "[Beethoven] represents the German dialectic, the incorporation of more and more notes into the scale, culminating with dodecaphonic democracy, where all the notes get an equal hearing." (p. 440) If one played all the Schoenberg quartets (as the WSC does at their party), beginning with the D major string quartet (1897) and ending with String Quartet No. 4 (1936), a progression from lower to higher entropy would be traced.
Definitely a nod to the great jazz club, The Five Spot, located at the corner of Cooper Square and St. Mark's Place, in New York City a very small club, where the tables were very close to each other and to the small stage where the musicians performed. Artists performing at the original Five Spot included Thelonious Sphere Monk (who played at the club when it premiered at its new location in 1957), Ornette Coleman (In November 1959, he brought his pianoless quartet Don Cherry on cornet, Charlie Haden on bass, drummer Billy Higgins for a controversial six-week stay playing his white plastic alto sax!), Charles Mingus and Cecil Taylor. There was also, in Manhattan, The Half Note, a jazz club located at the corner of Hudson & Spring Streets, known for its showcasing of up-and-coming jazz musicians in the 1950's and 60's.
"V Note" is a clever and Pynchonesque! name choice for a jazz club in V.. Both "V Note" and "Five Spot" are slang for a five dollar bill (or five-pound note), plus "V" connects to the novel's title, plus "V" represents the fifth degree of the musical scale (the "G" note in the key of "C"), plus a pointer to Blue Note Records, one of the most renowned jazz labels, whose artists included Ornette Coleman. One of the great pleasures of reading Pynchon is parsing these many, um, multivalences.
Not sure about "McClintic" (perhaps an old school or Navy buddy of Pynchon's it happens), but Sphere likely references the legendary and groundbreaking jazz pianist and composer Thelonious Sphere Monk (1917-1982). Also, in the beat/jazz parlance of the day, where cube or square denoted someone not hip to jazz and current beat culture happenings, "Sphere" would denote the opposite someone in The Know. On this topic, also read: Is Sphere Monk?
59/55 - He blew a hand-carved ivory alto saxophone
Likely a nod to Ornette Coleman, who played a white plastic alto saxophone. When he premiered his pianoless quartet at The Five Spot in Manhattan in 1959, shocked the jazz world with his extremely "free" approach to jazz harmony, structure and improvisation. And, indeed, he played with a sound "like nothing any of them had heard before" (V., p.59):
- Even from the beginning of Coleman's career, his music and playing were in many ways unorthodox. His approach to harmony and chord progression was far less rigid than that of swing or bebop performers; he was increasingly interested in playing what he heard rather than fitting it into predetermined chorus-structures and harmonies. His raw, highly vocalized sound and penchant for playing "in the cracks" of the scale led many Los Angeles jazz musicians to regard Coleman's playing as out-of-tune; he sometimes had difficulty finding like-minded musicians with whom to perform. 
Also notice that ivory "is a term for dentine, which constitutes the bulk of the teeth and tusks of animals" . This may not only be a reference to dentistry, which appears a number of times in V., but unlike normal saxes, which are made of inanimate metal, an ivory sax (however fictitious) is carved from something that was once animate.
59/56 - The group on the stand had no piano...
Reinforces the connection between Ornette Coleman and McClinic Sphere. Whereas, during this period, piano was a standard component of the jazz ensemble, Sphere's quartet, like Coleman's, has no piano. Where McClintic has his "natural horn" player, Ornette's other horn player, Don Cherry, played a "pocket trumpet," a scaled-down version of the instrument not typically associated with jazz (like that "natural horn"!), but through which he established his own distinctive style and timbral quality.
59/56 - a boy he had found in the Ozarks who blew a natural horn in F
Between the years 1936 and 1937, after his embarrassing attempts to solo at several Kansas City jam sessions, Charlie ("Yardbird" or "Bird") Parker (1920-1955) traveled to the Ozarks to work with the bands of Ernie Daniels, George E. Lee and "Professor" Buster Smith. In the Ozarks, Parker spent long hours woodshedding honing his technique. He took all of Count Basie's records, from which he learned all the Lester Young saxophone solos. At the end of this marathon woodshedding session, Parker reemerged as a mature player to be reckoned with.
A "Natural" horn is what all horns were before valves were invented. Their available pitches were limited to the natural overtone series, hence the term "natural" horn. You could obtain a few other pitches by moving the hand around in the bell, but that resulted in noticable changes in timbre. So a natural horn in a jazz ensemble would be quite something!
60/56 - There were people around, mostly those who wrote for Downbeat magazine or the liners of LP records, who seemed to feel he played disregarding chord changes completely
In Down Beat, George Hoefer described the reactions of the audience at a special press preview of Ornette Coleman's quartet the Five Spot in 1959: "Some walked in and out before they could finish a drink, some sat mesmerized by the sound, others talked constantly to their neighbors at the table or argued with drink in hand at the bar." 
"liners of LP records" refers to the notes on the back of LP ("long-player") records, talking about what's on the record and how great it is.
60/57 - He plays all the notes Bird missed...
Reinforces the connection between Ornette Coleman and McClintic Sphere. Coleman, as noted above, had a penchant "for playing 'in the cracks' of the scale," which led to many musicians thinking he was playing out of tune.
In which Benny Profane, a schlemihl and human yo-yo, gets to an apocheir
The Whole Sick Crew
In which Stencil, a quick-change artist, does eight impersonations
In which Esther gets a nose job
In which Stencil nearly goes West with an alligator
In which Profane returns to street level
She hangs on the western wall
In which Rachel gets her yo-yo back, Roony sings a song, and Stencil calls on Bloody Chiclitz
In which various sets of young people get together
Confessions of Fausto Maijstral
In which things are not so amusing
In which the yo-yo string is revealed as a state of mind
V. in love