Elmer style ring seems to only occur on the smooth base with June 17th 1890 patent date and no mold line over base. It is shown in top most photo. Bottom most photo shows the above listed embossing variation with plunger pushed into dome glass ( if I can say a 251 has a dome). One can see shape of threading plunger end.

Round base and smooth base w/ mold line over base both don't have much happing at top of pin hole

Smooth base without June 17th 1890 date seems to be only N.E.G.M CD 251 with pennyquick style teat.

I'm curious about what the inside of pin hole looks like on hhe CD 250 though.