It has been my experience, numerous times, that finding a used plastic buttplate to fit a used stock and having the "fit" acceptable is a long shot at best. For a good fit you most likely will end up sanding it with the butt to get everything acceptable. Sometimes you get lucky and will find an original plate that is still proud of the wood so you can fit it properly. Often though it will be shy on one spot, above on another etc etc. I've had far better luck using repro plates, which are made oversize (usually) and fitting them to the stock, especially if you have an old finish you want to maintain. Google gungrips.com or nc ordnance (same place now). They make decent quality repro buttplates, pistol grips etc. For the buttplate, best thing is to block sand the butt end of the stock lightly as well as the backside of the buttplate itself so they will fit together nice and tight. Screw it on tight, then take a pin point sharp pencil and mark the stock line all around onto the buttplate. Remove and block sand down to that line making sure you follow the correct angle, like on the toe. Test fit on and off as needed, otherwise for best results sand it attached to/with the stock, but then you will need to refinish the stock and the forearm so they match.