Not a lot to go on. F could be Fabrica or Fratelli, (the latter meaning brothers). The initials are probably a founder's initials. The last letter S could stand for Silma, but other names might include Sabatti, Scotti, Serna, Sierra, and Sosso. My suggestion, if you're still wanting an answer, would be to approach either the company named on the top rib, the gun's sellor, or finally the proof authorities in Italy, giving them all the numbers and letters you can find. The first thing I would do, however, would be to look closely at a new Silma in a shop or email their importer. I've seen them for sale here, so I'll let you know if I come across one again.