The M39's basis on the 76mm GMC M18 is apparent when the running gear is compared. The turrets have been omitted in favor of an open-topped cargo and troop compartment. (Picture from TM 9-2800-1/TO 19-75A-89.)
The arrangement of the front of the vehicle is illustrated here. The drivers were positioned under the low roof in front of the superstructure. The headlights are missing from this vehicle, though their guards remain, but the siren is present in the center of the front hull.
Stowage baskets were added to the superstructure sides and rear, and stowage compartments are visible along the hull sides above the tracks. (Photo by Richard S. Eshleman.)
The engine access arrangement and towing pintle were carried over from the 76mm GMC M18. (Photo by Richard S. Eshleman.)
The rear deck of the vehicle is shown here. A. Water can retainer. B. Hull air inlet grille. C. Oil tank filler screen door. D. Hull rear roof door. E. Hull air outlet grille left. F. Fuel tank gage cover. G. Hull air outlet grille center. H. Fuel tank cap cover. I. Hull rear upper plate. J. Hull air outlet grille right. (Picture from TM 9-755 76-mm Gun Motor Carriage M18 and Armored Utility Vehicle M39.)
The rear of the troop compartment is shown in this image. The machine gun ring mount is to the left of the picture, and the interior space for crew and stowage is obvious.