Gen 1, Gen 2 or Gen 3? Whats the easiest way to tell the difference between these 3 Glock versions? I purchased a Gen 4 G23 today & im looking to buy another but wanna buy used to save a few bucks and wanna make sure i know what Gen Glock im looking at. I asked the dealer and he wasn't sure unless it was a Gen 4 lol. Thanks in advance. Chris.