Second thoughts: it's for the pressel switch all right, but it's for use with the WS62 in the AOP role in Auster or Beaver aircraft. In that role a headset and throat microphone similar to the for the WS46 is used with the WS62, and it has no pressel switch so can't put the WS62 onto transmit. The 'T' connector shown is fitted between the headset/microphone and the WS62 droplead, and the flying lead connects to Terminal Block 8 in the aircraft, which connects to a pushbutton on top of the control column. The pilot and observer headgear plugs into the T connector and the second droplead.
The "Beaver" AOP aircraft has a VHF set as well, so there's an additional control box to allow both sets to coexist with the headsets.
I've seen the headgear assembly on eBay in the past, and was somewhat puzzled by it: it's a standard 5-pt droplead plug, with a throat microphone (low level, No.2) as used with the WS38 Mk.I & II and WS46, and a Y cable with earpieces but no headband - obviously for use with a flying helmet but it initially made no sense at all!
Anyway, Wireless for the Warrior Volume 2, pages W.S.62 - 28-30 have some diagrams and explanation.
I really don't know what we'd do without Louis Meulstee!
Chris.
(Pleased that I no longer need to bid on them.)