The 2X2 is an EHT half wave rectifier. We also have the later 2X2A vibration resistant variant in the museum.
The 2.5 volt heater indicates that it would be powered by a special heater winding of high insulation. Such a valve would have found application in oscilloscopes or in multiplier chains for higher voltages.
The Ken-rad maker name on the base cap.
Inside the bell the coated cathode is supported only at one end to give good isolation from the anode.
The classic envelope is 38 mm in diameter and, excluding the UX4 base pins, is 96 mm tall.
References: Data-sheet, 3002 & 1043. Type 2X2 was first introduced in 1939. See also1939 adverts.