The image shows a 250cc M.S. motorcycle as raced by Sandford in the June 21 1921 Grand Prix de France, which he won. M.S. may mean Moto Sandford. The official records for the race state that the motorcycle is an MS-Train.
Sandford also raced Sandford tri-cars, Morgan three-wheelers, and sidecars. In 1922 Stuart Sandford won a side-car race on a 350cc Metro-Tyler at the Montargis GP de France. The Sandford tri-car was first introduced in 1922 in Paris. Designed and built by Stuart Sandford the Sandfords became highly successful at time ...
(Apparently no relation to Richard Sandford VC)
La Moto Francaise, racingmemo.free.fr