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Tube sets were generally designed to overscan so that there was room for shrinkage as parts age or tubes weaken. It was a definite attempt to keep from showing black edges when everything got to the other end of tolerances. Broadcasters complained about it for decades, but TV manufacturers knew that owners would not complain about 10% overscan, but would generate a service complaint for even a small underscan. You're looking at typical design, I think.
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