The old .44 Top Breaks sometimes had brown grips. Looked like plastic but were gutta percha or some other older material. Would look like Service Style round butt grips.
I've seen a lot of them on old Smiths, but I don't think they were really "plastic". Maybe gutta percha or bakelite? Can't remember if they were on an N-frame, though.
Guys, thanks for the info. I ordered a set. They really look like checkered Magna grips in the picture but no S&W markings. I have seen a lot of aftermarket grips for S&W but never like this. Will report when I get them.