All this talk about the dash-4 being the best, but nobody has stated why. Anyway, I have a 2.5" dash-8 that's so new, the ink is still wet on the box label. It was intended for special purpose that never came, but that's the way it goes.
I don't have the proper reference... But I've been told MIM started with the Dash 4
(e.g., MIM thumbpiece on the SB; Thumbpiece, hammer, trigger on the RB).
I've been told that the 686-4's frame was converted from Square Butt to Round Butt in August 1996, beginning with the "CBC" prefix,
and that the switch to the MIM trigger & hammer occurred at that time. However, I have not seen that documented in S&W
reference material, nor have I closely inspected a SB and RB -4, so it remains conjecture to me.