there should be a tag on the bottom of the servo cover, usually that is the first tag that goes missing.
next on the top of the C6 below the bell housing is a flat area that should have a partial vin stamp.
if it is from a mustang 71-73 then the first should be a 1,2 or 3, followed by an F the next digits will be the unit number
if the trans came from another ford then the first digit and letter will be different.
An H code had an FMX trans. Q or R or M would of had a C-6
if you are trying to see if the trans is original to a 71, then you want to see the 1F followed by the cars partial vin on the top stamped area.
from there you can work back the original tag on the servo cover.
would start with PGA or PGB
AH, AU would of been a 4 barrel
you want to correlate it with the 4th Vin Letter as well for the engine.
you can also look at the letter on the servo Cover above where the tag is suppose to be.
From Softest to Most firm
CODE ON COVER, APPLY DIA, RELEASE DIA, SEAL TYPE
N 1.93 2.92 Molded piston
S 2.00 2.86 Molded piston
G 2.04 3.02 O-rings
J 2.07 2.69 O-rings
L 2.07 2.98 Molded piston
D 2.08 2.98 O-rings
P 2.10 2.86 Molded Piston
H 2.34 3.02 O-rings
R 2.48 3.50 O-rings
the closer you get to R the higher the performance of the Transmission.
back then you would of seen a S or J on the cover the big block Cobra jets got R servos