AG 406025 certainly appears to indicate the special order 235 and would indicate the 405,024 engine built that year (numbering started at 1001).
Using this website http://home.znet.com/c1937/Prod.htm
and knowing that the 235 was only available on conventional HD and COE models, the potential number of 235's would be 92,221 1.5 Tons and 3,540 COE's. That's far short of engine number 405,024.
Perhaps the engine number only makes sense if you include the 235 engine production with the total number of engines produced for all models in 1941 which was 1,243,554.
Just my thoughts. Hope this helps and good luck with your project.

"Nothing astonishes men so much as common sense and plain dealing." - Ralph Waldo Emerson