Titled by the photographer as a '30 model 500cc OHV sloper, but it appears that the SF31 did not enter the catalogue until 1931. SF31 is a two-valve model, the SG31 is 4-valve.

Photographed at the Solvang Museum by Craig Howell Licence: CC BY-NC-SA 2.0

Ariel Motorcycles of the 1930s