Since I have the pictures in my computer I thought I would provide a little K22 evolution (after the 17-4).
In a nutshell the big difference between the 17-4, 17-5 and 17-6 is that they have different barrel lengths and barrel profiles.
(This can be said about the early stainless K22s as well.)
First you must understand that the older model 17's and 18's not only had different barrels but their frames are finished different.
Below is a top view of the pre war K22 (L) Early post war and Model 18 (middle) and K22 "Heavy" Masterpiece aka Model 17 (right).
My guess is in 1985 the Model 18 line was dropped because it required more man hours to finish ending with the 18-4 .
(it has more contour cuts on the frame to match the tapered barrel ),
(1986 to 1989) ...Realizing a 4" K22 was still needed they instead offered a 4" version of the heavy barrel Model 17-5 .
( 4" 17-5 shown below)
S&W also made some early stainless heavy barrel versions of the 17-5 (without full underlug)
and model marked them as 17-5's and 17-6's.
In 1990 The full underlug blue 17-6 replaced the heavy barrel 17-5 and was available in 4", 6" and 8 3/8" barrel lengths .
1989 617 introduction
1999 Model 17 line is discontinued .
My guess is the stainless gun was cheaper to make as it didnt require blueing.
Eventually "Classic" Model 17 and 18 reappear but with very different frame shape, barrels, front and rear sites , internal locks, laminate grips and MIM hammer triggers.
New "classics" also now share the same frame (with eachother) only differing in barrel length.
Current 617-6 shown below.
