Starting this off - I will modify as new info comes to light. It is not 100% (by a long shot), so if you know something, post here! 9x19 1st Variation - 9x19 on slide only (Austrian Proofed G17,G17L, G18 and G19, 1st and 2nd Gen) 2nd Variation - 9x19 on slide and side of chamber (Early 2nd Gen G17, G17L, G18 and G19) 3rd Variation - 9x19 on slide and top of barrel (All late 2nd Gen 9mm Glocks and all 3rd Gen) .40 I know that the .40 caliber Glocks had at least 3 variations. 1st Variation: was simply .40 on the slide and .40 on the side of the chamber. 2nd Variation: The caliber mark was changed to .40 SW and was marked on the slide and top of the chamber. The lowest serial number I have observed is a G23 SL prefix, the marking ended between the CCP and CER range. 3rd Variation: The caliber marking reverted to simply .40 on the slide and top of chamber. .357 SIG - two variations. I do not have any idea of the transition range. 1st Variation: .357 SIG on slide and top of barrel. CDZ-prefixed G31s are known to have this marking. These may be one of the rarest standard production variations to exist. 2nd Variation: .357 on slide and top of barrel. 10mm Auto - two variations. The first variant is within the double alpha prefix. 1st Variaition: 10mm on slide and side of chamber. To at least the UW prefix. 2nd Variation: 10mm Auto on slide and top of chamber. .45 ACP - Two variations 1st Variation: .45 Auto on slide, .45 on side of chamber (no Auto) 2nd Variation: .45 Auto on slide and top of chamber .45 GAP Note that prototype G37s were marked .45 GLOCK on slide. I assume the top of the barrel was the same. 1st Variation - .45 G.A.P on slide and top of chamber .380 Auto Unknown at this time, but I assume that markings will coincide with the above in the appropriate date of manufacture range.