OM 636
A home-made freeze plug from France. One shot shows a traditional looking brass freeze plug, so it would seem to be a freeze plug, at least. He sourced the brass plug from Mogparts, but apparently it is not the correct size, and one that would fit was not handy.
It is hard to say, but he made a plug out of flat plate, that much is clear. He used a hammer in the press to form the dome.