Hi
My 1941 Pattern 12 C60L has the same hole in the same location on the back of the cab directly behind the battery box. So I suspect it was a standard hole regardless of the body or use of the truck. The picture below is of the back of my cap before I re-fabricated the back of the cab. All the measurements were so I could reproduce them on the new back panel. The other picture shows the hole from the inside.
The old cab back is like all sheet metal I've replace stored away for future reference in the barn.
Your picture made me go out to the barn to check and I discovered that though I had put all the small attachment holes in the new skin I failed to punch the hole in question either in the back of the cab or the battery box, so I will have to add those to my list of things to do on the truck.
