The ARP34 is a screened variable μ pentode designed for radio receiver use. The frequency range of operation would be up to 30 MHz and it was probably used as an IF amplifier. The commercial equivalent is the EF39.
The ARP in the designation designates this as a pre-CV UK Army Receiving Pentode.
The reverse is plain zinc paint.
This is another example of the ARP34 with the zinc paint starting to flake off.
The classic envelope is 30 mm in diameter and, excluding the IO base pins, is 87 mm long.
References: Datasheet, 1040 & 1043. Type ARP34 was first introduced in 1939.