Getting back to the year of your tube, my 1957 CTC-7 has an RCA 21CYP22 but it has "60" stamped right on the center of the base cap, meaning, I think, that it was made in 1960, three years after the set was built.
I have seen other CYP's on eBay that had 57 stamped in the same spot, more evidence that this is the year of manufacture?
If your tube is an RCA maybe it has a similar stamp or date code, maybe in another location?
Note the tube in this set:
http://cgi.ebay.com/ws/eBayISAPI.dll...&category=3638
has 72-17 stamped on it, i.e. 17th week of 1972