I do not know about military trucks.
GMC trucks had frame markings through about 1947.
Domestic/civilian Chevrolet trucks, to my knowledge, did not have frame markings until much later.
Some states, like CA, might have DMV-placed frame markings on old trucks.
If life, and DMVs, we're fair, they would be able to show you documentation that Chevrolet used frame markings on 41-46 domestic/civilian trucks.
Maybe your DMV will be satisfied with the GM-documentation-page I showed you and the following documentation:
http://chevy.oldcarmanualproject.com/shop/1941/41csm003.htmhttp://chevy.oldcarmanualproject.com/shop/1942/42csm0003.htmhttp://chevy.oldcarmanualproject.com/shop/1942_47/4247csm003.htm