my best guess is Pfaltzgraff stoneware, but am not sure. Pfaltzgraff is mostly know for pottery dinnerware, but the company first made decorative vases they called 'stoneware'. I found a link to some similar marks, but the book US Marks on Pottery Porcelain and Clay by Lois Lehner, show many more variations. If you can find the book,, see pages 341 & 342. Just my best guess. Have fun searching. Old pottery marks like this can be very difficult due to the thickness of the glaze, causing the thickness to obscure details.
