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Many of the early 5BP4's I've seen are dual marked 1802/5BP4, so this must have been during the transition time to the RMA-EIA markings. I can't find any data on when the various 1800 crt's were introduced, but all the 900 series ones are shown in the 1935 RCA CRT Manual, and some 1936 literature shows the 1800 and 1801 crt's.
Darryl
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