

Berkshire Hathaway/Warren Buffett. (Though the name at first was not BH)

You're probably thinking of Buffett Partnership, an investment fund which he ran largely by himself, using friends' and family money as initial capital.

Berkshire Hathaway is a textile mill that Buffett turned into an investment company, but by then he had been a successful investor for over 10 years.

I'm talking about the fund that he used to buy BH with.

Charlie Munger helped out Warren a lot. Not sure at what point their partnership started -- but it was before Buffet was making a ton of money.

I believe he started working with Munger 20 years after he started his own fund.

...which was around when Berkshire Hathaway turned from a textile manufacturer to an investment vehicle.

It's hard to lump financials in with tech startups, though. Technically, when he Buffett started the Buffett partnership, he had lots of cofounders. The structure of an investment fund is a single managing partner (who makes the decisions) and then lots of limited partners (who put up the money). In the eyes of the law, they're all cofounders.

If you go by practical instead of legal definitions - who provided the know-how? - then Buffett's early partner was effectively Benjamin Graham. After all, it was at Benjamin Graham's firm that Buffett first learned about investing, and Graham served as a mentor long afterwards.

Investors/mentors are not cofounders.

He and Munger were friends and colleagues earlier than that. I think they were in contact in the 50's.

I'm not sure about that. If I remember correctly from the handful of biographies I've read, I believe they met personally later than that.
