[Charlie]

I believe you will find that the metal in question is not part of that tube. I have never R&R'd a 21CYP, but it appears that is nothing more that what holds the CRT in place.

If I am not mistaken, you lay that set on it's face, remove the nuts and clips in the four corners, and that metal ring will come right out of there... and the glass CRT will remain until you lift it out.

The 21CYP22 in one of my CTC-9's was replaced during the mid-60's... it now has a 21FBP22 in there. That same metal "base" you mention is what holds my 21FBP in there.