Hello Chris
The BRACKETS AERIAL CLAMPING No.1 T. ZA29558 is for the top of a vehicle roof, I think??, only reference I've seen for it is in WFTW. Appears to have been used in LCV's and ACV's 4x4 HP RCA/19, one of the aerial connections on the Wireless Sender RCA ET 4332 B includes the following, Connector Single No.46, Insulator W.T. AE Lead-in No.16, Adaptors No.2, 4 or 5, Main Rod Aerials as required. I'm assuming they are referring to 'D' Rods, as the other aerial connection on the RCA set is for a Dipole No.9 which requires something heavier than an 'F' Rod set-up.
The above parts are listed in WFTW as being for ACV's & LCV's, also 'D' Rods were originally used for WS 1, 2, 3, 7, 9, and 11; all old school, later the WS22 used 'D' Rods as a 34' antenna/dipole mast set-up for Ground Stations.
There is also a reference in the WS53 chapter, picture is poor, ZA number is different ZA.24136 and "it is mounted with 'F' and 'D' Rods to the insulator fitted to the roof'"
It also states in WFTW that anything over 18' (6 'D' sections) required stays, 18' is a lot of torque and the Brackets Aerial Clamping would have stabilized the 'D' Rod antenna base/insulator, keeping it from ripping the roof mounting plate out in anything stronger than a light breeze.(Used only when vehicle was stationary)
I set up the pieces roughly to give an idea of how they might have worked. The Mounting would have had to be insulated from the vehicle as it is clamped directly to the 'D' Rod.
Also the 'F' Rod adaptor would have had to be installed at least one length of rod above the stay plate; as you can not turn the thumb screw on the adaptor when the stay plate is directly under the adaptor.
Anyways sort of like this, diameter of bracket is 8 5/16".
Geoff