The 3BP1 oscilloscope tube has a medium persistence green phosphor and a three inch diameter screen. Actual maximum diameter is 77 mm.
The 3BP1 design dates to the mid 1940s but was a popular choice for home construction oscilloscopes up to the 1970s.The main image shows the electron gun, the etched identification and the internal colloidal graphite coating that acted as the final anode.
The 14 pin base has the pin numbers embossed on in the plastic. The correct orientation is given by the base spigot.
The curved face of the tube.
This exhibit came from the home made oscilloscope that is seen in the foreground of this image and was made by G3JMB. The photograph dates to 1976 and features three UK radio amateurs. L to R G3JMB, and sons-in-law G8LSD and G8GHH. The scope design itself was not a great success with limited bandwidth and a poor focus to the trace. It spent over 30 years in a cupboard in the roof.
The end window envelope is 52 mm in diameter, and excluding the base pins is 235 mm tall.
Reference: Datasheet. Type 3BP1 was first introduced in 1944.