Here's a photo of the peekaboo slot in my dead 21AXP22:
The white area is getter material that changed color after the tube leaked and let air inside.
With a little adaptation, it's possible to substitute a newer all-glass CRT for the 21AXP22. That's what I did with my CTC-4.
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html