CD is actually GD, standing for "Guard". So the inscription means: Corporal in the 326th Guard and Fire Company, United States Army Quartermaster Corps.
There is not a lot of available archival information on the Guard and Fire Companies (they are not yet digitized), but in general these units were considered Permanent Camp Units and the members were not always what we know as "firefighters". They may have provided other duties, such as manning the fire stations, manufacturing, installing and operating various fire apparatuses (suppression systems, alarms, phone boxes) and providing general guard duty.
If Fred was indeed fighting fires, there should be some documentation somewhere that shows his connection to a fire station (either before or after service) since most of these soldiers were recruited into their positions (i.e., to man a fire truck and provide response services). What you may find is that Fred actually worked a farm or worked in manufacturing of some sort - which would indicate he was more of a supporting role during the war.
Anyways, each camp, post, or station had base units attached to it that provided basic services such as firefighting, housekeeping, medical services, motor transport, etc. Fire Truck and Hose Companies were the larger units, and were stationed at the military camps. The smaller posts and stations were smaller units staffed by the Guard and Fire Companies.
