I'd give the guess that the 'S' & the 'F' are for adjusting the clock (slow & fast) , so I'd look for an english language maker (but not always) .
I'm also guessing that when the works is removed from the case , more will be revealed .
Can you tell if the face is porcelain , or baked enamel ?
Below is an image of one like yours , sold in the U.K. by Tennants (auctioneers , Yorkshire) on June 6, 2009 for $160 pounds .
It had the key & leather carry-case with it .