I have one that I have been aching to install on my SKS; it was graciously given to me as a freebie by the excellent CGNer Lineman
I never fitted the action to it before, but I painted it desert tan and once I tried to fit it it is a little too snug

I got no paint inside, so the problem is somewhere in there. I have not had the time to figure out where there is insufficient space, but I am going to do it between now and christmas
I had heard that these fiberglass stocks were not in fact military issue, they are chinese manufacture, however they were designed to replace cracked stocks for their SKS exports. Not sure if any of these ever saw combat.
If they did, good. They are not quite however "aftermarket".
Norseman, how much are you selling those fiberglass ones for? When I am done with the tan one I might want to try a olive-green stock