Correct Ross, well done.
Dennis, I have never seen one of these before, but I realised that the UK MoD also used them to secure a jerry can between the seats of their Radio vehicles.
I checked the part number, and it was the same as the one in our RPS.
You need a T bolt and a special wing nut, that mount to the bracket in the centre of the tub.
Assembled, it looks like this,

The T bolt and Wing nut stow here when not in use,

It fits in fairly well, and is retained by the angle bracket mounted to the floor. The clamp neatly holds the jerry can, as well as locking the lid closed.

I know that pretty much all of these were removed in service, but even my late 1970 GS had one, it still has the chain for the bracket. The 88s and 109s all had the bracket that fits in the middle of the tub, for the 109, this was not fitted to the civilian vehicles as far as I know. They were fitted in the CRs in 1959, and lasted up to the end of the 2As.
Mystery solved, I have always wondered how this worked myself.