David, allow me to add my two cents worth. As noted, there are a lot of smiths signing the Tomoshige name. Fujishiro has oshigata for the Ôei Tomoshige (3rd gen.) and the Meiô Tomoshige (5th gen.) Although the Meikan gives dates for some of the other Kotô smiths, there are very few published oshigata with these dates. This makes it very difficult to pin down exactly which Tomoshige you have. Also, your Tomoshige does not have the classic Kaga nakago-jiri, which adds another problem for dating. I am presently in the States and do not have access to my library in Japan where I have the Kashû Kotô Taikan, which has a lot of Tomoshige oshigata. Nonetheless, I would suspect that dating this sword will be very difficult.