Forget suggesting that “343 Industries” was the name of some Halo-related studio at Microsoft. It's a fact, with the group officially revealing themselves as part of the Halo anime announcement.

Interestingly, 343 are described as a “division” of Microsoft, not a development studio. Which makes sense, since having spoken with 343's creative director, former Bungie man Frankie O'Connor, this LA Times piece says the division “oversees all Halo products”. Not just games. Everything. Games, comics, toys, cartoons, the works.
(http://www.kotaku.com.au/2009/07/microsofts-halo-division-is-called-343-industries/) source

Microsoft is teaming with five productions houses for a series of seven short films called Halo Legends, reports the Los Angeles Times. The five production houses are some of the biggest names in the industry:

Bones, Casio Entertainment, Production I.G., Studio4 C, and Toei Animation.

The creative director overseeing the production is Shinji Aramaki, director of Appleseed and Appleseed Ex Machina. Microsoft has also roped in Warner Bros. to distribute Halo Legends on DVD and via digital platforms early next year.
(http://www.kotaku.com.au/2009/07/details-about-halo-anime/) source