What size is this Gray ST12 socket?

Went to an estate sale at lunch and scored some Gray and Indestro Super tools! Woohoo

One of the sockets I bought was labeled "ST12 GRAY" and no other markings like size etc. I checked the current catalog and could not find this part number anywhere. It's 3/8 drive and looks like a 12 point semi deep socket. Anyone have any idea what size this is? 12mm?

acswan said:

You could try it on an Allen wrench or socket if you have any, I would guess 12mm or 3/8(as in 12/32),just guessing by how there part #'s go, hope this helps

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Using your hex key suggestion, It seems like it is a 3/8 socket. Maybe the "S" in the "ST" is for Standard . . .