Old mack line up is confusing as hell.
At the dealer we call anything with an offset cab a DM. But it's more complicated than that. DMs were short hoods, and they were also called U models. RMs were the log trucks with steel, center split hoods. But RM688s could also be SuperLiners, which have conventional cabs with normal, though huge, hoods. And RDs are the most of the old mack dumps you see still on the road. But normal "R" models had hoods about a 2 feet longer and were usually tractors.
Also, the big thing about the superliner was that you could get the E9 500hp V8. But most were sold with I6s, the E6 300hp one at that, and you could get the V8s in any model anyway.
Also, it didn't really matter what engine you got, cause you could have mack turn any engine up to 900hp if you wanted. It would just void the warranty.
They also told Crane cams they "didn't know what they were talking about" in the late 90s. This is because Crane was brought in to fix reliability issues with the E7, as they were eating cams since the mid 80s. Crane told them they needed to redesign the block in order to fit a physically larger cam and different lifter design.
The E7 continued to eat Cams until it was discontinued in 07.
I could go on for days about weird, frankly dumb things Mack has done. Granted I have basically surface level knowledge of these things since I've only been around them for over a year.