I have 6 of these in my collection and all are stamped on the inside left of the plate, with various marks. I was lucky enough to make a chance find of over 20 of them a few years ago on the battlefield. Every single one of them was stamped on the inside. I think I still have the ref pics I took of the marks somewhere. I would say that this is a copy with a recent paint job, unless there are stamps in there that dont show on the pics. Some of the marks are faint, but mostly they are very deep and would show on the pics he has posted. Ask the seller for extra pics of any stampings inside the plate. If it was me and the seller said there are no stampings I would leave it alone. Most pics in ref books also show stampings inside the plate, but as always, you never know.
Andy