Here is a photo of the Brompton rear derailleur. I marked the bolt that is an axle for the square with the tweezers that grab the upper pulley. I also marked the limit screws to make it easier to understand how the derailleur works. The shift cable is attached to a corner of the square from above. I am impressed by the Brompton's simple, inexpensive and creative solutions in their details. It is easy to design, making incremental improvements in solutions tested and adopted across the industry. In many details Brompton people pound alone and those contributing creativity into the designs evidently communicate with those responsible for production.
Thank you for the additional picture. Very helpful. There are many ways to make derailleurs work and great to see another variant.