Right now, the tube is a big unknown. I do not intend to just plug it into a tube checker, because if it is gassy, the heaters will likely blow. I suspect that I will bring up the heater voltage slowly and monitor for excessive heater current... if I get up to 6.3V with normal current and a nice orange-red glow, I will then use a crt checker.
The pinout and voltages of the crt look about the same as a 21AXP22 so I suspect that the guns are very similar.
One thing that has me worried is that the tube does NOT appear to be fritted together... So how was the front panel attached to the funnel?. Indeed, quoting Peter Keller again:
"Corning Glass Works developed the frit-seal process which was first introduced in 1958 for the all-glass 21CY22 color picture tube." I hope that they did not use the same process that was used on the 15GP22s.
When I obtained the set, it produced a fair picture, somewhat dim, and convergence was always a problem. The previous owner left the side control cover off, and said that he often fiddled with the convergence, which changed during operation. Yikes! Hot chassis and all!
jr