This topic comes up once in a while,
Here is a link to the last time as it covered alot of the questions at hand.

M-17 4"

( 1985 ) The Model 18 line was dropped with the 18-4 ,(until its recent revival with internal lock).

(1986 to 1989) ...Realizing a 4" K22 was still needed they offered a 4" version of the Model 17-5,
at that time it had a heavier no underlug barrel .

Some transitional late 17-4's , early 17-6s and early stainess 617's ( sometimes marked 17-6)
are found with the heavy no underlug barrel.