The 5A/157D is high impedance screened pentode designed for usa a intermediate frequency amplifier.
The KB/FE identifies this valve as being made to specification K1001 or K1006, the B denotes qualification by a UK authority and the FE identifies the maker as STC, Oldway (Additional to Paignton factory 'F').
The reverse carries the CV358 UK Government procurement code.
Looking into the top gives little extra information.
The wide glass tube envelope is 29 mm in diameter and, excluding the IO base pins, is 84 mm tall.
