The same photo of 'Ulster Gem' that appears in the 1916 Totty catalogue (posted here) also appears in the 1917 catalogue. The contrast is a little different but I'm not sure that there is any more detail particularly discernible. You can check it out here if you like:
Blooms up to 6", yes. Sepal length, maybe not. We need photos of Ulster Gem with younger buds. Re- reading the comments under "BL Precious Porcelain": Ulster Gem was described as having stiff petals. BL PP doesn't. I note also Patricia's comment that she thought BL PP looked like a second-generation gigantea. It starts flowering early enough to be a Clark rose.