S&W introduced the 4" heavy barrel profile in 1986 in the Model 17-4 series. It replaced the Model 18 which had a slimmer 4" barrel profile. The 17-6 was introduced in 1990, the earliest -6 guns still had the non-full lug barrel as S&W used up the parts on hand. The -5 and -6 guns had the new yoke retention system and radius stud package.
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