The 4" Model 17-5 HB is an interesting model for a few reasons,
First it was a change in direction for S&W to make a 4" and 6" version of the same gun again,
I say again because the 4" and 6" K22 were originally the same gun with different length barrels from 1949 until the wide rib K22 supplanted the tapered barrel 6" around 1955.

The 4" tapered barrel Model 18 continued until the 17-5 HB supplanted it in 1988.

The early 17-6 HB had the same non shrouded ejector rod HB as the 17-5 until it was changed to a full underlug barrel in the early 90's giving the earlier HB version a fairly short window of production,
Incidentally the 17-5 HB basicly has the same barrel profile and thicker wide rib as the Model 14 where previous versions had a narrower rib that keep the k22,K32 and K38 close to the same weight.

Post some pics Im sure the comments will be flowing.

L to R Models 617, 17-6, 17-5HB, 18


Prior to the 17-5 HB Previous 3 barrel profiles looked like this