So after the last chapter:
-we get to see that Isshin Shiba was the Captain of Squad 10 before Hitsugaya and had Matsumoto as Vice-Captain and Hitsugaya at 3rd seat at that time.
-we learn, as well, that he was the head of the Kaien noble Family.
-also we get to see Aizen-Tousen-Gin, all three of them as captains testin out the Hogyoku.
So this flashback is a few years after TBTP(Turn Back The Pendulum) and around 30 years before the meeting of Rukia and Ichigo.
Let's see now how Isshin and Masaki met!