The XFR3 was in live production by Hivac in the mid 1950s. It is a directly heated amplifier triode designed for dry battery operation. Hivac advertise the XFR3 as an RD Oscillator Valve. The Hivac range of miniature wire-ended valves were used in hearing aids in the 1950s and early 1960s. The CV number indicates that these valves were also used in equipment used by the UK government. Portable personal radio communications would be the obvious application.
An alternate view.
The thin glass envelope is 8 x 5 mm in section, and excluding the lead-out wires is 42 mm tall.