Cool find!
It is a 1939-42 GMC AFKX-352 shop van. I think most all were actually produced before we entered WW2. Roosevelt could see the writing on the wall...
The front drive axle was produced by the Timken Corporation.
There were several other body configurations made on a similar platform. It is basically the early version of the wartime CCKW and some parts interchange.
http://olive-drab.com/idphoto/id_photos_gmc_afkx.phpYou are lucky to see one.