I have seen them in the positions noted. but that was non-AC.
A – Power brake booster, red (manual cars use only two side holes on booster, bolts thru other with plate on firewall)
B – Steering column – 4 sheet metal screws
C – Washer hoses (small hose on top)
D- Engine feed wiring harness
E – Speedometer cable
F – Clutch rod
G – Emergency brake (also clutch spring return bracket on manual cars)
H- Accelerator pedal grommet
J- Accelerator bell crank – 3 sheet metal screws
K –Brake line clip
L – Fuse block, 2 sheet metal screws
M – Unsure – not used on either of my 68 S codes or 68 C code, maybe 6 cylinder use
N – Pedal support to firewall – 2 sheet metal screws
P – Speedometer Cable Clip
Q – Alternator Wiring Harness
R – Firewall Pad Retainer
S – n/a
T – Heater – 4 studs w/stamped nuts w/sealer (heat/vent only, not A/C cars)
U – Heater Hoses
V – Brake line clip
W – Firewall pad retainer
X – Unsure, think it may be dimple for 6 cyl accelerator rod bracket, not used on my V8’s
Y – heater motor shield, used on A/C cars and left on many big blocks
Z - Engine ground strap