[Bob Carriere]

... the back of your cab.... behind the driver...... as this rectangular hole......
( you will need to loose that modern rear window)

..... which might indicate that it was a wireless radio truck...... that hole was for a canvas rectangular tube between the hard cargo box and the driver.... so they could shout instructions..... so you may have a wireless truck..... which might explain the RCEME rebuilt engine late into its service life...mid fifties....

Bob C