If your truck is like my 71, you can get an idea of build date based on the date codes on the block and intake, for example. Nearly all date codes on this were a month prior the build month. Tells me things were working quickly, flow of inventory like it's supposed to be. May not apply to other situations.
A few more hints for perhaps build months.
My 400TH transmission is stamped 4 -71, as well as 19. This is likely a date code.
I suspect this 4-23 on my frame rail is the month and day, just a guess.
Take a peak at your brake pedal. Might be E-brake, if I remember the brake pedal has a date code.
70C-10 mentioned axle. My axle has a bunch of codes, but I haven't seen a date code. That said the ring gear is stamped 9/71, which to my truck means it was replaced soon after the truck was purchased. It's that or an entirely new axle soon after, hard to say, I wasn't there
