Stamped Numbers In Sheet Metal

[mrein3]

I've been doing some painting on my truck.

First let me start by saying my truck is a mutt. 1971 chassis (long wheel base 4x4). 1972 Blazer th350 and NP205 transfer case. 1972 Chevelle 307 temporary power plant. "New" to me box made up of pieces I bought in Wisconsin 10 years ago plus a re-pop LHS bedside I picked up from daburb. 1968 Golden anniversary interior.

NOTHING matches.

My normal operating mode with old sheet metal is to remove all the old finish, treat it with a mild acid wash, epoxy primer, then color.

On the back of my cab, and on the tailgate are stamped numbers. "39" on the back of the cab and "47" on the tailgate. The font appears to match so I would guess this was some factory marking.

Has anybody else noticed this? If you have a non-mutt, do the numbers match?