Hi David
(Not sure where SMHEA fits in with military vehicles, but there were apparently 2 SIIA FC and one SIIB tested by the ADE and the RAF used them in the UK)
In regards to your #053A vehicle, does it have rear body extensions to overhang the rear of the chassis? The other SMHEA SIIA body does and apart from the cladding on yours and the front corners the bodies seem to have similar design.
Can you find any evidence that the front peak of the body, particularly the corners, may have been modified?
Diana