From A.P.2548A, Vol. I, Chap.1 I found the following discription.

218. Sockets type 173 (Stores Ref. 10H/423) fitted with an early type of brass insert, are apt,
under vibration, to fall out of engagement with the external aerial ammeter used on the H.F. ranges
(Stores Ref. 10A/12227). Such inserts can be identified by the fact that they finish about 1/4 in. short of the moulded socket body.