If you look at the picture on the side of the box, that lower left clear part is a bed cover that fits between the rear of the camper top and the tailgate. There are two dimples in it where the hand rail fits in.
Based on my experience with antique Mack fire equipment, it appears to be a 1938-40 Mack E75S pumper. These were equipped with a 750 gpm pump and a booster tank in the 200 - 300 gallon range. The three louvers on side of the hood were replaced six (two groups of three) beginning in 1941