…The problem is, there is no room for the reel in a W113.
There was a service bulletin from MB to install retractable belts which involved cutting into the B-pillar behind the doors. Quite elaborate. Some owners have installed the reel under the boards in the cavity behind the seats. Concern is the movement of the belt from out there. I have been looking into this issue for 20 years and still do not have seat belts in my car.
When my car was restored in 1999-2000, I provided the technical service bulletin from Mercedes-Benz North America to the restorer. He bought the parts from MB and did the work without any issues. The bulletin is from May 1989. The seat belt kit was from a sedan and carried part number 114 860 23 85. Some other parts are required as noted in the instructions.
The reels are hidden and they look and perform as if they were designed that way from the start.
From a driver and passenger POV, perfect function. Zero complaints.