Here are the differences.
The G23 is only out there in 2nd and 3rd Gen.
The 1st gen style is only around on a G17, G17L, G18 and VERY VERY VERY VERY VERY few (like 10) G19s.
The 2.5 Gen style is only around on the G20, G21, G26, G27, G28, G29, G30, G33, G36 and G39.
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