I thought this was pretty cool. I don't remember the M-designation for it off the top of my forgetful head....but you don't see them too often. At least I haven't.
Looks like a "Gichner USN MB-2 Fire and Rescue Foam Truck" pictured on page 19 of TM9-1840C.
According to that manual: "In 1955, 130 M56 trucks of the flat-faced cowl without winch configuration (no cab or top) were supplied from government stocks to the Gichner Iron Works for Navy MB-2 fire and rescue foam trucks. These vehicles weighed 6200 lbs. and carried 160 gallons of water and 11 gallons of foam agent. The foam pump was operated by a Tangen power take-off located on top of the transmission (the gearshift lever extended all the way through the PTO). (For reference-the USN registration numbers were 71-00662 through 71-00791.)"