i have seen ca. 1944 sporting guns from "suhl und zella mehlis werke". me thinks one should not be able to tell that chambers have been lengthened by looking into them.
many ole' sauer game guns in 12ga and 16ga are light enough so there is no real plus in getting one chambered for 20ga. low grade old sauers are pretty much "dime a dozen" guns. i would say no deal.