Ken Scharf wrote:

It's a 'head on' photomultiplier tube.
They were used in flying spot scanners,
and noise generators in radar jammers.

No, it isn't a PMT.

The 1P25 was the first commercial infrared image converter, now commonly
called a "night vision" tube. Infrared energy is focused onto the
photocathode at the large end of the tube, and a green image appears on
a phosphor screen at the small end. These were used in "snooperscopes"
and "sniperscopes" during WW2.

Firing it up requires about 5,000 V at a very low current. A resistive
voltage divider sets the potentials for focusing, etc. The tube was
declassified shortly after the war, and a good overview of it's
operation was published in the Sept. 1946 issue of "Electronics". If you
can't get a copy, email me and I can scan it for you.

Bob Weiss N2IXK