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.
Our other National Union 2X2 has no mica support for the anode. This exhibit has a top support micas and grips the glass at four points.
Within the anode bell can be seen the coated cathode tube. This has a small diameter in relation to the bell as the emission required is low but the gap needs to be large to avoid breakdown when a high voltage is applied.
The classic envelope is 38 mm in diameter and, excluding the UX4 base pins, is 97 mm tall.
References: Datasheet, 3002 & 1043. Type 2X2 was first introduced in 1939.