The 5A/102D is an audio output pentode for telephone repeater amplifiers. The 5A/102D was the prototype for the CV1724 and both designations are printed on the glass. The old GPO code is POVT183.
See CV1724 for details.
The balloon envelope is 44 mm in diameter and, excluding the IO base pins, is 109 mm tall.
Reference: Data-sheet. Type 5A/102D was first introduced in 1946.