Very interesting item. Can you see joint marks near the marie? If so, how many? If no joint marks are present, this was likely pressed in a shell mould that gets flipped 180 degrees (after the glass sets up) so that the "catching out boy" can get it placed on a wide paddle. Then, it's stuck up on a large diameter punty rod with a flat head (the diameter of the head would be the same size as the area that is now seen as ground and polished). Once it's stuck up, it's reheated, iridized with a spray of metallic salts, and then taken to a skilled finisher to be cupped (hence, the stretch glass look that happens when an item is iridized BEFORE it's finished to its final shape). Then, it's cracked off the punty rod and taken to the lehr. Once annealed, that bottom area can be ground and polished. Fenton made a lot of plates this way in the 1920s-30s. I hesitate to speculate as to a maker, but it could be someone like Pairpoint or Durand in the USA, although I suspect its origin will turn out to be European.