The Winchester patent date pad would be an acceptable period pad to use. e rex is correct in that few Model 12s of that era had recoil pads, mostly just Trap and Skeet models. Yours has an aftermarket pad, and the one I suggested is actually from a little later on, ('40s to mid-late '50s) but would be the most correct of what's listed. The Winchester Trademark pad is from the '60s on, but could be used also.
If you wanted to be more correct, a buttplate would be most likely what it shipped with.