This is a Mack B81SX. It has the older B-series cab with flat fenders. This style of Mack predates the DM eries.
DM series had the R model cab that debuted in 1965. DM cabs were off-set to the left, B cabs were always centered on the frame.
mackinac359-the R models and DM's didnt use the same cab because R models have a vent on the cab in front of the door and DM's have a vent next to the hood and above the fender