FOUND! ARVIN 9219-cm-uhf television.

[dieseljeep]

Originally Posted by Yamamaya42

I guess I can make some assumptions here.

I believe you said the was a rebuild and aluminized, making it a 21FP4C, you also mentioned that “it has a self focus gun”, thus making it unique, according to what I find when I research the tube type.
https://frank.pocnet.net/sheets/093/2/21FP4A.pdf
Meaning it was rebuilt with a more modern gun assembly, having auto-focus, and needing no ion trap?

Am I correct here?

I'm pretty sure it needs an ion trap! Try it first to see if you get a raster. If so, it doesn't need one. If it doesn't, try an ion trap.
Of course, you have to remove the magnetic focus unit.
The focus pin is one that you have to without a connection or grounded or to B+ source. Most of them never needed a connection to provide proper focus.