That is a vulgar fraction and notes a diameter. Most of the time a 12 bore will be stamped 13 or 13/1 but I've seen a few that were stamped 11. In 1904/1905 there were articles that stated that S,D&G was sole Sauer agents and that the frames were 1" longer than typical. That's why I was curious as to the frame length.

Kind Regards,

Raimey
rse