Hi David,
Assuming the upper and lower ceramic insulators are 'stepped', then the hole in the mounting plate should match the smaller diameter of the insulator, and there will be a rubber or leather washer under the upper insulator to keep rain out of the vehicle.
(There will also be a square gasket that goes under the mounting plate for the same reason - I'm not sure what (if anything) they did to seal the mounting bolt holes, unless the square gasket also did that.)
I've got the British equivalent of the roof insulator, which is a ceramic mushroom of considerable size and weight supporting Aerial Base No.3 to provide flexibility for use on the move. Because Base No.3 was never intended to be fed from underneath (and was probably not up to the task for WS53 or ET4336) it's bypassed electrically by four braid straps and the mushroom does the insulating job. That lot mounts via a Tufnol plate (or to a steel bracket behind the cab on the "wireless light" vehicles) and I suspect the plates and gaskets are similar in external dimensions.
Chris.
(Not going anywhere (except possibly back to bed) at the moment, I have a foul cold.)
