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If you are referring to the Austrian Eagle and Nitro Proofs, at some point during the 2 letter runs during 1990 Glock no longer proofed the guns in Austria as Georgia had "come on line" for more tasks. Substituing the little Glock symbol on the slide and removing any marks on the frame. As I understand it only Glocks for the US market are so marked.
Early on Glock in Georgia was little more than an address and warehouse facility.
I have a KK###US which has both set of proofs.
I saw MM###US which did not.
Also as an aside, the real early Glocks had the Eagle/Nitro proofs on the slide directly over the eagle/proof mark on the frame. My G19 DS###US has that as do all my first generations. At some point they moved the marks on the slide to from in front of the ejection port to behind it where the current marks still reside.