found on http://truquetructruk.tumblr.com
for comparison, the Ford Model T of WW1 http://justacarguy.blogspot.com/2011/12/1917-ford-ww1-military-ambulance-none.html
When the U.S. entered World War I new impetus was given to Model 16 for military use, and it became the Class AA military truck for the duration of World War I. Of the 13,316 Model 16 truck produced, the major portion build in 1917 and 1918 were for military use.
The U.S. Army developed a classification of military vehicles and chose preferred supplies for class. GMC was selected to be the primary source for Class AA, ¾-ton trucks. The military version was designated as GMC mode 16AA. The most common application was for field ambulance service.
The company's booklet titled "GMC Ambulances" reads: "It is to the credit of the United States government that in every phase of preparation for the great war the utmost consideration was given to providing comfort for the soldiers; not only during the preparation term at the various encampments around the country, but in providing for their welfare on the field of battle.
"It is a source of much satisfaction to the General Motors Tuck Company that the Medical department of the United States Army found the GMC 3/4 – ton chassis to meet the requirements of this very exacting service."
The Model 16 had a 132-inch wheelbase with 35-by-5 pneumatic ties. The 30 horsepower engine gave the truck a max speed of 25 miles per hour. Although most of the ambulances were assembled by GMC, demand soon exceeded the production capacity of GM's Pontiac division facility, so kits were devised that included all of the parts required to build an ambulance. Some of those kits were assembled by subcontractors in the U.S. such as Huppmobile and others were built by American troops in France.
info from http://www.military.com/veteran-jobs/career-advice/military-transition/gm-military-history-motorized-vehicles.html