My '71 ain't a '71...or is it?

The number can be on the top or on the engine side of the radiator support.

Anywhere from near the battery to about the center of the radiator.

Occasionally they are found under the antifreeze sticker.

You will be looking for a number followed by a letter & then 6 more numbers with a space after the first number, they likely start with R1 in your case.

(edited to fix a typo & diction)