The 3RP1A is a three inch electrostatically deflected oscilloscope tube produced from 1955. It has sensitive deflection and a flat end window viewing screen. The 3RP1A has a medium persistence green phosphor.
This example was made for Daystrom. Daystrom were the original owners of the Heathkit brand and this tube Type was used in the Heathkit IO-21 oscilloscope from 1961.
The tube was made for Daystrom but the maker's name is not present.
The B12A duodecal base.
The base end and gun assembly. The deflection plates are hidden by the colloidal graphite coating inside the bell. This coating is connected to the final anode.
The optically flat end window - a feature usually only found on profession equipment tubes in the 1950s. The A variant refers to the flat face, the 3RP1 has a semi-flat face but is otherwise identical.
The IO-21 General Purpose Oscilloscope - image from ebay.
The end window envelope is 35 mm in diameter, and excluding the B12A base pins is 220 mm tall.
Reference: Data-sheet. Type 3RP1A was first introduced in 1955. See also1955 adverts.