I have never seen a square cylinder head and thought this to be very cool. Bidding on eBay can still be had for another day or less. Reserve not met at $2,700+.
“This is a highly collectable, 1953 Ariel Square Four – The Ariel Square Four was a British motorcycle designed by Edward Turner who devised the Square Four engine in 1928. At this time he was looking for work, showing drawings of his engine design to motorcycle manufacturers. The engine was essentially a pair of ‘across frame’ OHC parallel twins joined by their geared central flywheels, with one four cylinder block (or Monobloc) and one head. The idea for the engine was rejected by BSA, but adopted by Ariel. Thus it became the Ariel Square Four…”