Hi Dave,
I've come frustratingly close to that engine number: WR3925879, which was in a C15A, registration 141321, and all around your number, but not
IT. 
The records, particularly for longer-serving special vehicles (as workshop trucks were), are overwritten to the point of being almost illegible, so very difficult to track down. Still, the above gives you somewhere to start looking in AWM126 while you while away the hours under lockdown!!
Regards
Mike