Seen some oversize buttplates (maybe just at toe or heel which would suggest cross-grain shrinkage of the stock in the larger dimension) but Win m12 "SKEET"guns reasonably often (the handful I've seen and two of the three I own) display Winchester factory buttplates undersized all round but generally concentric to the section of the butt. I have never figured this one out but conjecture that they break and get replaced with takeoffs in better condition and that the actual profiling of stocks may change from batch to batch (pattern stock replaced) or from era to era (the model 12 definitely goes in one era and out another!!) and the buttplate from one yr may not fit the butt from another. Of course with handwork getting out a stock AND finishing, all bets are off about what will fit what.
On the other hand the deterioration of synthetic polymers as described above may be sufficient to explain these perfectly concentric, undersize buttplates.
