The Squariel.

The Old Lady needed a bit of shopping today, so I wandered the 100 yards along to the local food emporium and was confronted by this dream.

An Ariel Square Four.

The original 1928 version was very modern having an overhead cam 500 cc engine; a couple of years later it was bored out to 601 cc. They had a reputation for the rear cylinder pair overheating, a problem in common with other in-line engines so it was redesigned and re-appeared in 1937 with a 996 cc engine and telescopic front suspension.

The machine went through a series of incremental improvements culminating in this, the last production model which ran from 1953 to 1959. It featured and alloy engine replacing the original cast-iron version (significantly reducing the weight) and a "four pipe" exhaust system. Wikipedia describes it as having "four separate exhaust pipes" but, as you can see, the pipes join under the cylinder block; this has the effect of smoothing the airflow through the exhaust thus evacuating the cylinders more thoroughly and also making the engine quieter.
This was a genuine 100 mph machine

I started motorcycling in 1964 and would have given my eye teeth for one of these things.

I still have fond memories of driving down the motorway, in the rain, from Embra to the Midlands (there was a woman involved, now Mrs TD) and stopping at service stations for petrol. The person manning the pay kiosk would solemnly take my soggy pound note and hang it on a line along with all the other soggy pound notes from all the other soggy bikers.