Round stamp marks on Starrett 13 double square

I bought a 4” Starrett 13 double square recently that is in good shape, except that there are several round stamp marks on it, each of which are identical and which look like a circle with two rows of numbers and letters inside. Anybody know what they could be? The square has someone’s names scratched onto it, so I don’t think the round stamps marks are necessarily for identification.

(I know, I know, I should include a picture, but I won’t have the opportunity till tomorrow.)

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