Here's a preview:
The back of the set: the first clue
1956 date + 21AX = second clue. Isn't it interesting how they included the original installation date of the set (maybe the date the CTC-4 was built into the custom cabinet?) and the date the CRT was replaced on what had to be a 1961 sticker for the repair?
1967 Service tag. Likely an RCA Factory Service job; note the "Cherry Hill, NJ" on the tag. This may have been for a second CRT replacement. If so, this tube is probably pretty "low mileage."
A view of the innards.
Another view of the insides. You can see how the cabinet was extended laterally to allow for storage of the tambour doors when opened.
And finally a closeup view of the neck of the CRT. It looks pretty silvery under all that dust. <crosses fingers>