This thread has been helpful, but there is a lot of "it should fit", and "I've bought it, but haven't fitted it yet".
So, I have done some research on my own, and happened to have doors from a Mercedes W123 wagon in the barn (1984 230TE).
The stoppers from that car fits the coupe like a glove, with only minor modifications.
The arm is 80mm long, and the coupe original is 85mm.
Can't imagine the door opening more than it does now.
They're perfect, and look stock too.
I ordered a set of new ones for $37 on eBay, and the part numbers are:
Mercedes: 123 720 05 16
Meyle: 014 072 0034
I had some plastic vacuum tubing that fit the new stoppers, and made new bushings by fitting it and drilling a 5mm hole to open it up.
Since I don't have the pins, I used M5 allen bolts and lock nuts.
I had to grind away some metal on the stoppers to get the correct angle on the arm, but other than that it was a great replacement. No grinding to the arm itself.
See attached pics.