That is a type 5890 tube. A seriously obscure device that is essentially a 5TP4 electron gun firing into a 2X2A style anode. I stuck it in there to load the CRT filament winding. and give a visual indication of filaments lighting before I installed the rest of the tubes.
Datasheet here:
http://www.shinjo.info/frank/sheets/049/5/5890.pdf
The 5890 was a sort of grandparent to the 6BK4 or 6BD4 HV shunt regulator tubes. It was used as an HV regulator in RCA theater/industrial projection sets. You could connect the second anode to the cap, as well, if you wanted to load test the HV circuit without a CRT installed. I have a pair of them kicking around the shop, and they are also useful for testing a CRT tester.
