Model 39 came out in mid 50s.
All the 2nd gen models ended in the late 80s and had about a decade run. Most can be identified by their 3 digit name.... 639, 659...etc. Then came the 3rd gen updated models with 4 digit names.....5906, 4006, 1006.
S&W naming is a little confusing. Each number stands for a option, finish, trigger, material. So a 5906 would be a full size 9mm, double stack, all stainless steel double/single action but a 5946 would be the same gun but double action only. However these aren't hard rules. Some 3 digit numbered guns were also 3rd gen suck as the 910 which was the economy version of the 5906 but made during the 94-04 ban era so it would only have a 10 round mag.
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