You are correct Your not allowed to make them any more. The RPB semi open bolt macs were made back in the late 70's early 80's and were only made for a year or two until BATFE deamed them machine guns any guns made after the ruling had to be registered as machine guns. They grandfathered all the ones that were made prior to the ruling. So there are not many out there but they do exist and are considered collectors items. There was also a tech nine open bolt also.
It wasn't so much that they were so easy to convert it's that you could make them go FA with out any perminent mods or even any modifications to the gun at all.