IDEA!
Is it an early version of the "Aerial Base C2" used on wireless trucks?
The one shown as "Fig.9" & "Fig.10" on pages 41 & 42 in the Wireless Sets, Canadian No.52 Vehicle and Ground Stations book (ZA/CAN 0077) from May 1944 looks as though it could be a development of this. (It's certainly got the threaded rod that goes through two ceramic insulators - on on each side of the roof plate - with one nut and a wingnut (with an optional extension rod for certain installations similar to the British "Aerial Lead-in No.16").)
May have been for Wireless Set No.9, the precursor of the WS 52?
(I suspect it was too rigid for use on the move with 'F' rods inserted, and the main purpose was static use with 'F' rods or to provide a feed to the telescopic mast (20-ft or 34-ft) and/or wire aerials in a static role.)
Best regards,
Chris.