I did consider the "S", I have checked my collection and I have a Large WM 28th London (Artists) pre 1936 with the same reverse (no "S") as yours and it looks its age.
I also have a large pre 1922 Leinster Officers spun back button with the HOBSON & SON LONDON back mark.
Was there a period when Hobson just had the one son early in the twentieth century?
Rob
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