Ariel square 4 engine set in a BSA A10 frame with swing arm rear and front fork suspension, with better brakes, electric etc etc. 
A recent build of great quality, clearly by a very experienced expert. 
 nice paint, rides well, the best of both worlds the amazing square four engine set in one of the best frames available at the time. Looks great too. 
The engine is the Mk 2 with the four port head giving better cooling, also the updated oil pump. 
The bike has been built to a very high standard, with all new paint and mechanical parts.