Buttplates are a frustration for me. With castings, they naturally vary. The biggest frustration is that they are bent coming from the foundry. I personally straighten every buttplate. So if one doesn't fit, you know who to blame! Anyways... we try to get them as straight as possible, but it's difficult to get them good enough that they fit a cnc cut stock inlet well.
I've contemplated various solutions, but none are easy. One idea I had is to make a handful of steel forms shaped like the buttplate cut-out of the stock. Each buttplate could be clamped to a form and then heated to allow the stresses to relax and then fit. This would be a lot of work, however. Another idea is to make a die where a heated buttplate could be inserted and the die closed on it. This would be faster, but the die would be pretty tricky to make well. Nothing is easy!