George,
I think that you are getting confused between the MkIV LRC and the MkIIIA. The MkIIIA was the last version produced on Contract S1993 dated 20/1/43 for 774 vehicles.
The MkIIIA has a fully floating rear axle unique to this vehicle, it also has an improved two stage air filter. The MkIIIA has triangular 'pistol-ports' on the front corners of the hull. It carries a spare wheel and so to make room for this the large rear stowage box has been split and mounted just in front of the rear wheels. All MkIIIAs were originally stowed for service in the Middle East and as such has extra water cans, sand channel brackets, sun compass bracket and a radiator condenser.
The engine, gearbox and transfer box are identical to the MkIII.
I know this, because I own one.
Please visit my webpage
http://www.asox18.dsl.pipex.com/eqpt.../humberlrc.htm
Les.