Some of the big companies did remove the passenger seats for insurance reasons, I know of a couple excavating companies around here that did that.
I owned quite a few DM's over the years.
Here is my former #56, a 1993 DM690S, with 300hp Maxidyne, Mack T2070 7 speed trans and 44k Mack rears Notice that the drivers side of the cab is pretty much even with the drivers side fender, and the dump box:
Here is my former #59, a 1991 RB688S with 400 hp Mack engine, Mack T2130 13 speed trans and Mack 44k rears. Notice that the cab, although the same as the DM cab, is not even with the drivers side fender or the dump box, but is centered:
Here is a sales brochure photo of an R model interior:
Here is a "classic" DM, that was owned by a company I worked for back east. It's a 1969 DM611SX, with ENDT673C 250 hp Mack engine, Mack TRQ7220 Quadruplex trans (2 stick 5&4), and 55k Mack rears. This truck did not have "spring brakes, or Maxi brakes for parking, it had that drum type parking brake on the trans output shaft, as discussed earlier: