User blog comment:Godisme/Everything But The Rain Op. 2 "The Rudiments"/@comment-1534479-20130307041834/@comment-5551986-20130307134255

@Smoshes I "think" Rukia was a shinigami some 100 years ago. Toushiro became 10th division captain sometime after Isshin dissapeared which was 20 years ago i think. Matsumoto didnt notice things about Isshin because Isshin was absent when Toushiro and Matsumoto went to his house. And as stated by Lemursrule he has aged considerably.

@Kiadote12 if i had to swing in the dark, delving into my vague memory or TBTP arc, Rukia may not have ever seen the 10th division captain., its unlikely but still has that chance of happening. She wasnt high enough rank to go to Division meetings at the Captain Commanders place, and Isshin seemed semi-lazy, so it would be understandable if he rarely left his Division HQ, he could have been like the Kenpachi of that period. Or may have been too busy. BUT i think he became a captain some time after Urahara's disapearance, due to Matsumoto's age. I dont think every single seated officer knows every single captain. For all we know, that flashback could have been Isshin's first few months on the Job.