I am restoring a 1966 GT mustang back to concourse I have found several markings in the fender wells and can make some out like the - 41 on the right side frame rail just behind bumper bracket
Covered earlier in a number of threads but here is the first one that came up in the search. Numbers used were limited to only a few - not every one between 0 and 9
https://www.concoursmustang.com/forum/index.php?topic=2257.msg11576#msg11576I see what looks like A/C AND P and something after that and can make some numbers
Over or below black engine compartment paint? By your description guessing might be both
on the left frame behind bumper bracket that looks like 3 2 something and looks like a 4
See above for the frame rail markings .
on the rear end looks like a red 3 and a white check mark on the back
Rear ends and third members often have more than a dozen markings depending on the application. Guess you've already read the article in the Library about basic paint marks. Check mark on the back if the housing is not common on all
.I found the white dabs on the front coil springs but nothing on rear leaf spring .
White is not an identification mark (to an application) showing that the springs tested outside the required range. So as to not waste anything Ford would still use them but installed a "match set" on a car
Depending on when the car was built the rears would have had spray paint dots or stripes. The dots don't hold up to rust and age as well as the stripes typically
IF your going to reproduce what you found you'll need to document them well and for some of them determine how you will reapply them correctly. Way too many pictures out there where owners/builders applied some marks over paint that were originally under paint. There is a thread on the site where I go through some of the steps I did preparing the bare metal, applying the crayons and overcoating them with paint when I resprayed the radiator support. You can barely make them out depending on the angle
Good luck