That would be an 913 CRT, as used in Millen modulation scopes (as well
as a somewhat obscure pre-war military radio transmitter). One of the
X and one of the Y plates are tied together.
--
Will
My father had some ham radio related device with a one
inch CRT in it. That basically looked like a 6L6 with a white top ? including having an
octal socket (8 pins). That?s even harder than the device you described.
paul