Hi Jon!
Some information coming up… for your friend. Watch this space!
Later.
Tell the new owner to check out a map of Takahashi (in Okayama today), formerly the Matsuyama Domain ruled by the Ishikawa, and later the Itakura Daimyo.
Takahashi City is overlooked by the fabulous little Matsuyama Castle. The district and street names reflect its feudal history, including Ishibiya-Chō where guns were made.
The Mei reads Takahashi Ishikawa Chūzaburō Saku
高梁忠三郎 作
Probably mid-Edo period, mid 1700s, the lack of a lower name suggesting he was the founder of this line.
The ‘buku’ pan repair was commonly done in Japan with well-used guns, blackpowder flash being corrosive to the iron pan, so my first and strongest inclination would be Japan. It is cleanly done, so possibly carried out abroad by someone who had discovered the old process, but I would be somewhat surprised if it was.