Rob,
Have had a quick look through Jeff Clew's book 'The Best Twin' and found in Appendix 3 for gearbox symbols 1919-37 the prefix FG states 3 1/2hp ohv (with new pattern support arm and kickstart support) The S1 was a 3 1/2ho model 494cc. The previous prefix for this model was AG. I'm sure some one else out there can enlightening us further!
Reg.