Bolted. And I do make 20" wheel trays as well. Could order an extra top bar and then a full set of 20" trays and then would only have to pull two bolts to swap entire system. 4-6 mtn bikes to 4-6 bmx bikes in two bolts time.
Also, that was first try at the bolt on trays, completely different now! And he wanted his rack as fully welded as possible, so no swing down option and his top bar is welded on as well... I'm hoping to make then burly enough to avoid ever having to fully weld them again as if there's ever a failure of one small part three entire rack doesn't have to be replaced!

 I saw you made different trays. The extra top bar is a great idea. I would still want to be able to mix just in case I wanted to take both. I'll be out in June, hope to see one in person.

 Cool! Just gimme a heads up and I'll arrange it!