User blog comment:Lia Schiffer/397: Edge of The Silence/@comment-1248963-20100328060104

@ItzBlitz: When Isshin revealed his status as a Shinigami, after killing Grand Fisher, he talked to Urahara, it was then that Urahara said something around the lines of "How does it feel to have your powers back after 20 years?" this implied that Isshin had been a Shinigami in the past but for an unknown reason lost his powers 20 years ago.

My take for Isshin being from the 10th division is the 20 years gap. Toshiro is a young captain and we know he is the most recently appointed of the Gotei 13. We don't know for certain when he took captainship for the 10th, but Ichimaru and Byakuya became captains about 40 years ago, so it must have happened after that. 10-20 years ago seems a plausible time. The 10th division has some mistery about it. It's captain had just died during TBtP. And I'm not fond of the Zero Squad theory because it seems sooooo predictable.