I have an old Model 56 with a badly pitted barrel. Still shoots okay, but I would like to get it either replaced or refinished. Saw a Model 57 barrel on eBay the other day, and wondered if the parts are truly interchangeable. Seems that the primary difference is the tubular magazine (57) vs. the stick mag (56). Also, I need a source to replace the cracked plastic buttplate. Thanks fo rthe assist!