A few clarifications I know from the 1975-1980 realm (Dan still gets a cookie though for setting the curve in class.)
B43: Nonpower brakes - Disc Front/Drum Rear
C15: Quiet-car package (sound insulation)
This is the minimal package/ next step up from stock, compared to C17 which had package tray, floor, door, etc sound insulation in it.
M51: Sunroof w/vinyl top - This is listed as the power sunroof option and M52 is the manual sunroof option up to 1978.
L73: from 1975 to 1977 was the "Fuel Pacer" system , which is wired into the driver's side turn signal (typically shows on the driver's side fender mounted light)...if it's still functional it will "flicker" if you give it too much gas, and if you are still "into it", it will light solid until you let off to the proper level of 'pedal' then it will go out... (at one time I had heard this got calibrated to about 6-9" of vacuum...which is about where the power valve tips in on a Holley 1945...)
I have seen cars that had the equipment removed, and the drivers side fender light just stayed lit... much to the dismay of the owner who couldn't understand why and thought it was a short....
Cheers!