Hawker Typhoon cockpit test socket.

What is the function of this test socket located in a clip immediately above the Type 359 M/T Socket location on the Starboard Electrical Services Panel?
There is no evidence of it or its wiring routing in the Wiring Layout diagrams of A.P.1804A. Vol 1. Sect.6, figures 2-14.
Any information would be appreciated.
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