The 3DP1 is an industrial electrostatically deflected cathode-ray tube with a medium persistence green phosphor. But it also has a radial deflecting electrode in the centre of the 76 mm diameter face plate.
The design use is for deflection with polar co-ordinates so that trace length can be accuratly measured.
The attached data-sheet for the 3DP1A dates to 1945 and supersedes the 3DP1 thus dating this exhibit to probably the early 1940s.
The RCA logo Type designation and 14 pin base.
The face plate with the centre radial deflecting electrode connection.
The end window envelope is 52 mm in diameter, and excluding the base pins is 245 mm tall.
Reference: Datasheet. Type 3DP1 was first introduced in 1942.