Quick update: Got the pad cleaned by soaking overnight in hot water with a lot of "Dawn" (grease-cutting) dish soap. Most of the cosmo wiped off, but some was hardened to the point I needed to use a firm toothbrush. That got it all.
It's also the end of the good news. The pad, which was paid for and shipped as "NOS," is absolutely a well-used item -- you can easily see the indentations on the "forward" end where it meets the wood stock. The butt/rear side is very heavily pitted and gouged, which was camouflaged under the thick coat of cosmo.
Have sent an email to Apex, but five days later still no response. I've bought a lot of stuff from Apex with no problem, although they sell (or sold) a wood foregrip claimed to be for a Sten Mk.V, which is most assuredly is not. When I asked to return that item they sent me a very nasty email. Decided to keep it and move on. I think the buttpad will be my last Apex purchase.