I have been quietly collecting all "Pre-RKE" and Edmund RKE variants I could find. There are quite a few similarities from the Sherman Tank M-70 gun sight war surplus eyepieces on the right to the modern RKE's on the Left... The Edscorp Kellner, as well as a few other post WWII surplus lens marketers, would sell this eyepiece as a Type III Kellner or as we call them today, a Plossl. I have done quite a bit of research on this particular eyepiece and it's evolution.
EDIT: If anyone out there is reading this and they have a similar eyepiece that look like that it could fit into this "family tree" please contact me via PM.