If you think missing tracks was just an artifact of the quad era you are sadly mistaken. It happens just as often today with 5.1 and Atmos. The new Atmos
American Pie left off
all the electric guitar leads. That lame enough?
As for
Angry Young Man, pretty sure what happened there. They booked a studio for the remix that lacked enough board inputs for all the channels and reverbs. This shows up in many early Columbia or Epic quads as reverbless instruments (a glaring example is the dry, hokey synth in
Great Suburban Showdown on
Streetlife Serenade). The entire quad
Turnstiles is a mess, what with non-master takes, off-balance parts, out-of-tune horns (a first), and dry (no-reverb) drums. But that missing piano part takes the cake as arguably the lamest missing instrument in multichannel history (Billy Joel
without his piano?) It only occurs in the middle section where all the acoustic guitars are, so I’m guessing they just ran out of inputs. The engineer probably had three days to do it, and found out only on day two that the board was insufficient. Rather than get bitched out by Columbia, he just did the best he could. Nobody ever checked his work anyway. We know this because someone would have dropped dead of a heart attack if they had.
As regards alternate solos like the famous second half of the organ break in
Do It Again, I’ve never found a single word in any interviews with artists or engineers mentioning this common issue. But I believe the only explanation for why they are no longer available is that they were live punch-ins performed during the stereo mix, done as a last-minute change after the track was complete when no additional tape track was available.