My father in law passed away last week, one of the old photos he had was him and his twin brother with their parents on an old sidecar. It's estimated the photo was taken around 1935, the original is black and white, but someone has kindly coloured it for me. Can anyone make out what the bike is from the photo?
As above it appears to be a twin, do we even know it's British? Many of that period were singles so should narrow it a bit. The forks and the sidecar suggest a 'competition' trials type machine of the era.
There are some clues; cylinder barrel shape and position in frame - very clearly vertical. Exhaust header curve and position of join where it meet the pipe. Front of tank shape. Look at the bikes you mention from that era and nothing matches these points.
The front mud guard has been removed leaving just the stay pushed up most likely so as to avoid debris being trapped, likewise the sidecar has no mudguard - a lot of the roads were probably rough. Front headlight missing. Nearly all bikes of that era had girder forks.
Don't think Vincent ever made a parallel twin, though potentially it could be a 500 single as we can't see the other side very clearly, but if it is the exhaust is on the wrong side. Girder forks are often associated with Vincent as they stuck with them long after everyone else were going telescopic. The lack of headlamp/mudguards and the angle of the forks/handlebars made me wonder whether it was some form of off road competition bike, you would certainly get filthy riding it! As Clanger says it could well be a bitsa.
The single cylinder Vincent engines were effectively the front half of the V twin unit so the cylinder should slope forward. Back in the early 1960s I used to ride a Vincent Comet complete with girder forks. That bike was about 10 years old at the time.
I don't think it's a Vincent, it's too upright and the exhaust is on the wrong side. I think it's a parallel twin, if it isn't it could be an AJS or even a Jawa to support my competition theory!
Most British singles had their exhaust exit down the right side so suggests a twin, although may not be British! If a twin, Triumph is the most common. It would be a pre-unit so casing would look a little different. Still think it may be an off road competition bike so could be a combination (no pun intended!) lots of early bikes were made up of bits from different manufacturers.