We worked with the Seabees in the first Gulf War. Good guys.
One of our heavy equipment operators knocker over an aluminum street light at an airfield.
Seabees raised it back up and repaired with a weld.
I would call that an "axle mount" derailleur.
I have used them successfully on beach cruisers and muscle bikes.
That screw behind the axle must be kept tight or you can end up with a trashed derailleur and possible some broken spokes.