I suspect the grommet is a water seal that goes under the aerial base No.8 or 10 on a vehicle roof. It's shown in EMER FZ256/3 of 1st October 1944 for the Canadian WS19 Mk.III, but I've not seen where it's supposed to be fitted. Probably need a late set of installation instructions for one of the "house type" radio bodies to be sure - they had a flat plate on a stand-off from the roof (two or three on each side) and I have an adapter plate to convert a 6 hole mounting to 4 bolts for the 'B' set aerial base mounting. I would imagine the grommet plugs the hole in the roof and is high enough to stop water getting in - the central hole is far too small for co-ax and I think Aerial Feeder No.9, which is P11 rubber covered multistrand like the battery leads with a ring terminal on one end for the connector plate under the aerial base and a plug to mimic the end of an aerial rod for the set end.)
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