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Sometime in the 70's a federal law came into being about picture tube size. Suddenly they were measured by "viewable area". This is how a 25" set from the 60's (25xp22 or similar) is the same shape as a 70's 23" set with a tube number starting out "23v" for "23 viewable". Later 25" sets had squarer corners on the tubes (not the same as the old 25" or 23" viewable sets).
A not so obvoius part of this confusion is that new roundie tubes made after that law became "19v" for 19 viewable. Reman tubes carried their original number. So in the 70's there were 21" and 19" roundie tubes that were exactly the same size.
I have seen one, and after looking it up in the RCA tube manual from about 1975 or so, It was not listed as a black matrix tube (though some rectangular tubes were). So if some RCA 19v roundies were black matrix, apparently not all of them were. The tube I saw was listed as having equal-brightness phosphors, though. A theoretical increase in usable brightness, even if it wasnt black matrix.
Your mileage may vary....
John
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