US 20x110 HS (electic M24) TP M99"RIAMC" or "R 1 AMC"

Anybody to tell what “RIAMC” or “R 1 AMC” means?

Image from the web:

Alex, it is the ammunition identification code (R1AMC).

Fede, thanks!
I am more used to modern ones so did not recognize this one.

BTW: do you happen to know when ID codes were introduced?

Group Code “R” was introduced with the Standard Nomenclature List (SNL) as early as 1927 and the complementary AIC was added in 1942.

Fede, thanks! That is earlier than I thought!