The HL23 is a low power low impedance low gain, battery triode. The data-sheet describes the HL23 as suitable for a range of application areas within a receiver including AF amplification.
This exhibit is unusual in that the commercial HL23 has the Air Ministry Logo on the envelope. Normally the UK service procurement code of VR130 or CV1130 would be present without the commercial Type designation.
The green zinc screening paint carries several pieces of information. Here the circle contains the letters CLW and 17.
The old stores code of 10E/159B is present.
The as well as the Air Ministry Logo is another circle with the letters CLW and the number 7.
The classic envelope is 32 mm in diameter and, excluding the MO base pins, is 70 mm tall.
References: Data-sheet, 1040 & 1043. Type HL23 was first introduced in 1938.