Rather than filling the holes and rendering them useless for all time; find a gunshop/gunsmith and get 4? of the little screws made specifically to fill unused mounting holes. Brownells, major sporting goods stores should all carry them. might even get lucky at a decent hardware store.
They should be #6-48 and about 1/8" in length.
At one time here in the mile high cowtown; every store had small packs for sale or a smith in the back that could dip into his supply. Sports Authority, Gander Mountain, Bass Pro, Sportsman's Warehouse, ad naseum......
