Found this thread while researching a Tsuba I just won on auction. A nice little Tosho Tsuba signed "Tetsujin". I found some more info thanks to the previous posters in this thread.
There is mention of him and his family in the following book :
Classical Weaponry of Japan: Special Weapons and Tactics of the Martial Arts
By Serge Mol
His family were not only skilled martial artists but also iron craftsman, producing swords, swordguards and son. It appears that Aoki "Tetsujin" Kanesada and his brother both signed there work "Tetsujin". It goes on to say they posessed metal-working skill of the highest order and that objects made by them were of exceptional quality.
The Aoki family also continued the line of Enmei Ryu (a style of martial arts) which was also known as Enmei Jitte Ryu, Tetsujin Jitte Ryu, Tetsujin Nito Ryu, and Tetsujin Jitte Ryu. And by the way, Enmei Ryu was the style originally studied by Myamoto Musashi, the style founded by his grandfather.
Also, we have a date of 1643 as to when Aoki "Tetsujin" Kanesada wrote some illustrated manuscripts. So, we have a good date for when these may have been made. Interesting stuff.