Ariel Square four engine. I think this is one of the later ones which had improved cooling of the rear cylinders | Sketches, Motorcycle engine, Scooter bike
Stock Stories: Ariel Square Four - Hagerty Media
Ariel Motorcycles!
Ariel Square Four: The Squariel - Old Bike Australasia
Square Four Motocycles !!! - YouTube
The Queen's Revenge Austin Motor Scene
Ariel Square Four MK1 1949-53 exploded engine view with part numbers 9.5 x 14" | eBay
59 Ariel Square Four Photos and Premium High Res Pictures - Getty Images