I noticed that too and assume it is because it is easier to work in steel. The pivot bolt has been cut and threaded and a new part has been made that screws into the pivot bolt to sandwich the stop. That piece threads into the drop-out. I have to say, the design is so elegant that I do not immediately think the seller is wrong. I am open to the possibility that it may be a factory prototype or even just an R&D piece done as a test that was never even considered a prototype for any real production. Simply, a "how are we going to incorporate a and adjustable parallelogram angle" type of exercise done in the factory machine shop. It's possible.