The CRT might've replaced at a later date.
IIRC, those CRT's were available as late as the mid-60's.
They were rather pricey back then.

A thought from Daves brain.
 

The tube has been mounted in the cabinet all this time and is still good. Would it be prudent to leave it alone and not handle it, lay it on its side, etc, or put and new or different stress on the infamous metal to glass joint that seems to be the achilles tendon of the 15gp CRT. Seems like short lived and failures seem to happen after installs, storage, swaps, etc.
This tubes condition seems truly remarkable for its age. Just sayin.

If you use ordinary care when handling it, I wouldn't be too terrified that it's going to suddenly fail. The two 15GP22s that I got with my CT-100 have survived being handled & moved around during the course of restoration -- not to mention riding from central California to Washington state in the back of a moving van when I bought the set. Last year, I bought a TM-10 monitor with a 15GP22 and that one is fine after being shipped halfway across the USA on a pallet.

Phil "knock on wood" Nelson

Phil, did you get your CT100 from a guy named Dan in Oceano?

No, a guy named Len.

Phil Nelson