Glad to help!
Have had the two for many years. If HS was the first to make, then I know based on the age of my first 527. Other than that, the stocks are the same, both with full-length aluminum chassis, appear to come from the same cast/mold. I've had CZ Varmint makes with wood stocks that required inletting and magazine fit fixes, but neither Kevlar version required altering except bedding the tang and re-bedding the lug area, which is something I do/personal preference.
A few things:
1. I very much doubt that a typical Varmint Kevlar owner is going to know whether his stock was made by HS or B&C.
2. Am not confident a CZ Kevlar owner would part with a Kevlar stock. Been out of production at least 10 years, and we're not talking 870 production volume.
3. If the B&C website is accurate (showing availability), I'd jump and never look back.
Another idea is call the guy that runs Stocky's. He used to sell the aftermarket stocks...might have some useful insight.