That looks like an XR-2 model. The XR-1 model had a rod in the hand linkage instead of the rectangular bit. Also, if I remember right, on the XR-1 there was only one chain going from shoulder to elbow. The other side was cable just like from elbow to wrist.
The XR-3 enclosed the encoders in plastic cylinders (originally milled aluminum). The reason why is because if you set the arm near a sunlit window and turned on the controller, the arm would fold into what we called the 'fetal position'. The exposed sensors were IR sensitive.
The controller for the XR-1 was an open PCB that sat on top of two 12V power supplies. One supply for +12V and the other for -12V.
The XR-2 and XR-3 used an enclosed PCB. The controller for the XR-2 used an 8748 processor. A later controller used an 80188 processor and a bunch of 8051 to control the motors.
Then came the XR-4. Instead of milled open links, it used solid plate. It was blue anodized aluminum. It used the Mark-IV controller that had the 80188 processor.
The arms were/are produced by Rhino Robots. I worked for them from 1982 to 1990. I was Chief Engineer and designed the Mark III and Mark IV controllers as well as several accessories. When I left we had just starting making small slant bed lathes. Also did a wire cutter (owner saw an opportunity) and helped on the Scorpion - a mobile robot.
At the time, they were in Champaign Illinois. You can find them on the internet but it looks like they're in Ohio now. Still selling the same units!?
I still have an XR-1 arm. The controller is long gone though.
If you remember the movie 'Short Circuit', as the move begins, you'll see an XR picking up a phone.