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This is a 5SP7 cathode ray tube. The '5' means it has a 5" diameter screen, the 'S' denotes the type, and the 'P7' means it has radar-style long persistence phosphor that glows bright blue and fades to a yellowish color.
It's special because it has two electron guns. Some dual-trace oscilloscopes used such a tube to generate two waveforms on the screen at once. Most other oscilloscopes either draw the first trace followed by the second trace, or alternate quickly between the two traces ("chop" mode).
This particular one came in its original box which was packed and sealed in 1952. The pen is for scale and did not come with it.