I think those stamped numbers can't be the date of manufacture. I'd be more inclined to think that they're either a lug part number, or they indicate something like tubing size or lug size...something along those lines.

By the way, the upper lug on my '77 is stamped 75, maybe 76 (I guess I'll have to break down and get some reading glasses someday. Not this decade, but someday...), while the lower lug is stamped 61, maybe 81. So it can't be the date of manufacture, I would guess.