I have observed and read in a recognized manuscript that about
Sn. A180000-A200000 their occurred a frame change and a mag change then about A200000 the factory mags came with the black plastic follower. First their was a small cut added to the bottom front part of the grip strap to facilitate the shorter 11/2 in. base plate S&W stamped. But retained the skeletonized steel follower.
Any of you that have a 39-2 in the above Sn. range take a look at your mag and let me know what you have. Either post or PM me. thanks, Mike