Need help ID'ing window regulator 300Cd
 

There is a guy selling all 4 window regulators from a 1983 300CD. I only need the driver side rear. The guy selling them can not tell which is the left or right side regulator (nor can I) other then me pulling out the one in my car is there any way to tell them apart ? Possibly a part # difference or some easy way I can tell the guy which one to send me.
Thanks in advance

Left rear window regulator p/n: 123-670-15-03
Right rear p/n: 123-670-16-03

Visually, I don't know how to differentiate the 2 rear window regulators from each other, but the rear regulators are smaller, more compact than the front window regulators.