There are all sorts of theories about which squad Isshin was formerly captain/a member of. Tite Kubo has yet to reveal much of Isshin's history concerning the Soul Society.

We know:
He's a shinigami.
He lost his powers about 20 years ago.
He and Urahara seem to be friends.
He possesses captain-level strength.
Isshin and Aizen are familiar with each other.

The questions:
If he had been a high ranking officer or even captain, how come nobody in the Seiretei's heard of 'Kurosaki'?

-Main Question-
Which squad was Isshin in, or was he even in a squad?

So: What's your theory about Isshin?