Are the only ones I know how to decipher.

The first number, followed by a letter is the year.

The letter tells you the decade
A is 1930s
B is 1940s
C is 1950s
D is 1960s
can't remember if it goes further than that.

The first number tells you the year....

so 3B is 1943
2A is 1932
4C is 1954, etc.

A lot of them are on the front of the cards, as seen below. Sometimes they're on the back...or even in the stamp box!