Riiiiiiiight.... all becomes clear.
It fits on Aerial Base No.16, which consists of a (modified?) Aerial Base No.3 on top of Insulator W/T "H" (which is a substantial ceramic piece). This was intended for (relatively) high power sets (WS19HP, WS12, WS53, etc) and the Aerial Base No.3 was not up to the job of insulating that much voltage. So: the ceramic (W/T 'H') provides the insulation and feed-through into the vehicle, and the No.3 is bypassed by four braid straps and just acts as a vibration mounting.
This is fine for a few 'F' rods, but once you start stacking 'D' rods, the hollow rubber Base No.3 will collapse, so you need that bracket to support the weight of the vertical aerial. Obviously you can't use it on the move, and since it's only supporting the weight of the rods, it doesn't need to be fastened on (certainly not bolted down, anyway).
I'm currently cleaning up a Base No.16, and noticed it has four large flat-headed studs on the clamp ring, so that's obviously what the slotted lugs on the bracket mate with. (Drop it on top, rotate into place, feed your first 'D' rod in and tighten the clamps on base and bracket, then add more rods as required. (Or fit the bracket to the assembled vertical before lifting it into position, then slide down, rotate, and clamp in place.))
Once I've got it cleaned up I will see about some photographs of it.
Chris.
(And that bracket is something else to look for on this side of the pond.)
