It is a B grade SBT and the stock extension etc. won't hurt it a
whole lot. Those things can be rectified for the most part. After all, there were not a lot of SB's made and trapshooters, being a funny bunch, modified the fit of the stocks with all sorts of contraptions so there are probably not many pristine B grade SBT's out there anyway.
But please do something about the floor plate screws...
They're probably just in the wrong holes.