I know it's just a choice of language used, but I always called that type of plate an oval plate because of the way it differs from the classic round plate found for instance in some Tung Sol 6SN7GT black glass tubes for instance. Those plates really forms a circle emanating from and terminating from very close location in the tube. Brimar had made 6SN7s that look a great deal like the 6J5 you are discussing, but they were not called round plates. However just to add some murkiness to the picture, if that wasn't enough, Brimar has produced some CV1988 6SN7GTYs with colored glass and what I call round plates. I don't think it matters after all is said and done, the only thing that really counts is performance.
Having black glass or any other fancy attribute doesn't automatically confer some level of excellence. I know this for sure, I have some very mediocre sounding Tung Sol round plate 6SN7GTs to prove that point.