I was always under the impression that the CS9 magazine catch (part #262590000) which is shared with the 908 and 909 value pistols was polymer/plastic.
I have a couple of those #26259 mag catches in my spare parts kit. One is from many years ago, when I decided to order a "parts kit" spare directly from the factory for my then-new CS9, and the other one is from within the last year or so, when I had a chance to order another one from a retail company who sells new S&W parts.
Both are steel and can be picked up with a magnet.
The button/nuts, however, are plastic.
Ditto for a couple of CS45 mag catches, spanning the same wide time range and sources.
Now, I can't remember anything in my notes from various armorer classes that lists they ever used a plastic mag catch body for the CS & "value" line guns, but then again, with S&W
"never say never". They did make some former metal parts out of plastic in the 3rd gens over time (mainspring plunger, disconnector, sights & some guide rods).
I just haven't ever
personally seen a mag catch body made out of plastic. Just the nut/button.
Of course, it's seldom that a mag catch actually requires replacement/repair, meaning either the traditional threaded post bodies or the bodies on which the plastic buttons snap over the end of the post.
The simpler non-threaded, non-adjustable bodies didn't involve the use of the small plunger & spring in the separate hole under the edge of the threaded mag catch button, so they didn't seem to gather oxidation in tight places like might sometimes happen with the traditional mag catch design. Armorers were told
not to remove the plastic buttons from the mag catch except for repair/replacement, meaning no disassembly for routine cleaning or inspections. (Because the plastic buttons were a 1-time use/installed part, and if ever removed, it would be replaced with a new one.)
I've known my fair share of guys & gals who bought 908/457 & Chiefs Special models over the years, and none of them ever required replacement of the original mag catch assembly. That's why I never ordered more than 1 or 2 replacement mag assembly parts, and in actuality, those were simply ordered since I owned a CS9 & CS45 myself and was in the habit of adding to my spare parts when I could afford to order them. Just in case.
I eventually decided to have a couple spare plastic buttons for each spare mag catch body I put back. They were available, so why not? I don't plan to ever remove the mag catch in either Chiefs Special unless it's ever required for repair.
I've put a respectable number of rounds downrange through both of them, though, and have replaced the recoil springs several times in each over the years.