Hi there!
Well nice radio truck you have there!
1st thing 1st... as a educated guess the "S4" would be the 4th vehicle of that troop or section. I have photos of Aust vehicles in the Middle East spoting the same type of markings with the shots of "S" "K" "M" "H" "T" and "HA". They were all of vehicles assigned to Sigs.
The hook on the cab rear wall are for hanging radio head sets on.
I use to own a very sad example of one of these bodies some time back but it was only good for parts and fittings. But hope to help a mate soon to restore his.
Good luck with it!
Phil... 
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collection includes:-
Chev "BLITZ's" (CMP's)
Inc:- No.8 FGT, C8A HUW, C15, No.9 GCT (sold)
Milt Land Rovers 
Inc:- 58 "gun buggy", 60 FFW, 70 FFW, 71 10 seater Wgn, 69 GS.
M3 Stuart Light Tank "hybrid" 
FV1600 Humber FFW/cargo
Mk1 Ferret scout car (waiting restoration)
Various trailers
Inc:- K38, "Ben Hur" 1 ton, 200 Gal "Humber" water tank Tlr, Aust jeep, Landrover recovery.
Milt Radios
etc etc... 
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