Hi,
On my SU-COE the chassis number is on the LH chassis rail, just in front of the front wheel. All of the ID plates would have been inside the cab in the centre of the dash above the engine cover. This would only show the chassis no. and not the census no.
If you're lucky the military number may still be under the paint on the driver's and passenger's doors as per Morris C8's photo above. If not I'm afraid there's pretty much zero chance of finding it.
Do you own both the trucks in the photo or just the Presland one? That looks like an artillery tractor which would have had a winch behind the cab, whereas the one on the right appears to be the cargo variant like mine.
Regards - MG