A couple of comments. From years, and I point out YEARS of these collectors collecting, they have not seen grips like you have. The chance of them being real is slim.
I too sometimes go the what-if route, and question the "known" facts, but if many respected collectors tell me that something is probably fake, it probably is. There were over 30 years of what-ifs before you got the gun, so who knows.
I too would want them to be real, but this doesn't make or break your "black widow". Which incidently is a made-up term from America and has no basis in europe or the Mauser factory. It is simply a luger that got bakelite/plastic grips on it instead of wood. Wood grips would be correct also on this same gun.
Ed