With reference to your photograph on page 929 of August's Motor Sport, I think the "mystery car" is a Joswin which was a large German car of the early 1920s. The resemblance is in the radiators.
The car was fitted with a Mercedes engine which had overhead valves anti not sidevalves. There were two different engine sizes, one of 6.5-litres and the other of 7.3-litres.
Chesham, Bucks. SIMON...