Curt Teich postcards, right? (At least the linen ones from the 1930's - 1960-ish)

If not, here's the code:

A = 1930
B = 1940
C = 1950
D = 1960

Look at the number/letter combination that is the first 2 symbols of the postcard code. The first number is the year, the letter is the decade. For example 4A is 1934.

Often the code is on the front of the card, but sometimes it's on the back, sometimes in the postage box

On the example below (lower right corner) the code is 0C....so it's 1950

I agree the hunt is fun...as long as you find something now and then!