Definitely NOT a Silver Arrow (400cc). The rider's leg is not wide enough to conceal the wide block of a twin with cylinders behind one another in the one block, and there is no fore-to-back mounted magneto. Although a twin the Silver Arrow had only a single exhaust system. I know because due to the poor cooling they were prone to seize, as I discovered when the one I borrowed in Australia did just that and I ended up with the bike on top of me. Only the right leg, over the top of the bike, was burnt by the exhaust as there was - fortunately - none on the side which was pinning me to the ground.
Peter C.