500ER
It is/was the 500E with a C20R engine as an option.
The 500E is not that much different from a D if you read the TCDS, mostly cosmetic. Major difference would be the chopped and lowered bulkhead between the front and rear. Front windows, T-tail endplates...............whatever.
In fact you can fit the "R" engine in a D if you want. The "R" is/was renowned for running "HOT" and to see the difference you would probably need a magnifying glass on the TOT gauge.
The "R" engine has some good improvements from a technical standpoint but possibly not when you consider the economics. There are loads of "B" engines and parts out there.
The "R" has been plagued with niggly little things over the years and are not cheaper to overhaul over a B. Coking, Oil leaks, Fuel burn...................blah blah.
"R" supposedly makes 450 HP and adds 6.8 kgs.
"B" supposedly makes 420 HP (but not if you read the fine print)!