The 3rd gen numbering system took some cues from the 2nd gen numbering system. You notice there are 3 different designations for 9mm's (39XX, 59XX, and 69XX) and the rest only have one series. My theory has always been that because there was the the, X39, X59, and X69, with the X representing the frame material, in the 2nd gen pistols, they continued the three different series in the 3rd gen pistols. Where the 10mm, .40, .22, and .45 has to fit all of their guns into one grouping the 9mm had a "grandfathered" in system that they wanted to include in the new system.
The frame material confusion is still one big mystery though, because even on the 2nd gens a 639 and a 659 had a SS frame and a 669 had a SS slide and Alloy frame. This confusion just continued in the 3rd gen naming system.
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J.D. Roy
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