Glock mag question, no caliber marking.

[FreedomIsn'tFree]

I had a question for the Glock guys on here. I bought some used G19 mags that seem to be gen1 or 2 mags. 2 of the mags have followers marked 9mm3, glock markings on the body and floor plate, but no caliber marking on the body like my gen3/4 mags. The other 2 have nothing marked on the follower(different looking followers), glock markings on both the body and floor plate and again no caliber markings on the body of the mags. Is this normal for the gen1/2 mags to have no caliber marks on the body? All 4 of the mags are 10/15 mags.

 

[FreedomIsn'tFree]

After doing some research I think I may have found the answer to my question. It appears these mags were from when Glock first came out and didn't have a caliber marking because 9mm was the only caliber available.