User blog comment:Godisme/Pure Speculation Thread/@comment-1966603-20100412040942

Perhaps, but I think that's a little elaborate for the Central 46... Another theory of mine is that he was the Kenpachi that wasn't present at the ceremony for Urahara's promotion to 12th squad captain. This would explain how Urahara and Isshin knew each other. Furthermore, since Isshin's only spent 20-ish years in the Human World, it could be that when he gave up being a shinigami, Urahara helped him emigrate there by giving him a gigai and possibly some other items.