Hi Geoff,
I've got one of those, and think it's from the Supply Unit No.5 (the hand generator, 3 lead-acid cells, and vibrator pack all in a WS18-sized case) intended for the WS18 or WS38 (the latter may have been an afterthought since the supply unit is several times the size and weight of the WS38). Also that cable is far too long to be a battery adapter (if such a thing actually existed in the first place). It's UX5 (807 valve base) to B4 socket, and the WS18 battery socket is rather smaller than a UX5 base.
Most likely explanation is that if/when they ran out of the correct batteries for the set, they'd get a shipment of WS18 batteries and the radio mechanics would have to swap the battery plugs from 4 to 5 pin as necessary. (The obvious method would be to make up replacement junction boxes from stuff held in stores and issue those.)
Chris.
(I'm expecting to find all kinds of things that I thought were lost for ever while I'm busy moving house - there's an entire loft full of clutter for a start!)
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