***UPDATE***

So, I went ahead and swapped out the door check straps in both of my W123's and I got some answers. Both of my cars had the original check straps and both had the 100mm in the front and the 95mm in the back. On both cars, I replaced them with good, used original Mercedes check straps that were also 100mm in front and 95mm in the back. I also painted the arms on all the straps to match the car's body color. I greased the replacements and they work great now. It never really bothered me too much that the doors didn't stay in their positions when opened, but what a difference it makes when they do function correctly. I also took the opportunity to grease the window regulators, which is also a nice improvement, obviously.

I'm not positive, but I get the impression that the after market check straps only come in one size. I'm not sure if it's 95mm or 100mm though. It seems like either size would technically work for either door though.