The AXP in my CTC-4 is gassy so I used a 21FBP22 as a substitute, and I can tell you it does work.
Any 21" good roundy color jug will work. With the glass jugs you need to remove the plastic CRT cover, make a new HV lead/plug, unscrew all the screen purity magnets so they are as far from the screen as they go, and ground the CRT's dag. On the non-21CYP22 tubes(I can't speak for the 21CPYP as I've never owned one) you will need to put some thick plastic between the metal yoke shell and HV connection on the side of the CRT to prevent arcing(because the grounded yoke will practically overlap the HV connector) According to a previous post I guess that some collectors prefer to cut a notch out of the yoke shell instead of using insulation to prevent arcing.
I'm not going to try to find my thread now, but to give you an idea of how good a deal you got...I shelled out over 400 clams for my CTC-4 pictured here
http://s1095.photobucket.com/user/El...?sort=3&page=1 , and I have seen nice 4's on the bay go for north of a grand.
My restoration is on hold now as I moved and since then I have not been able to clear enough bench space to work on it....That and it has IF/AGC intermittents and alignment problems that I don't have the experience or equipment to properly address at present.
There was something else I was going to say, but I'm so groggy right now that I for got it as I was typing the last sentence...
EDIT: Bob's post jogged my memory. I was going to say the 21AXP22 CRT's are about as bad about going to air as the 15GP22....Only there is not as much of if any effort to create a successful rebuilding process.
PS: I'm saving my AXP and it's plastic insulator for the day when they can once more be rebuilt.