For preservation sake... I found the answer.

I called S&W and no, they never made a threaded midsize 45. My gun smith wasn't correct that it wouldn't work, it needs ever so slightly less space between the slide and the end of the threads to fit - but should fit almost all suppressors just fine. Apparently S&W had plans for one but it never made sense to add a whole different part numbers, but the guy I talked to said he had plans of doing the same.