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Social gaming company Zynga (ZNGA) yesterday filed for a $400 million secondary stock offering, three months after going public. All of the proceeds will go to “selling shareholders.” Beyond that, however, details are unclear.

Take a look at the following chart, from page 117 of the offering document (click to enlarge):

You’ll notice that every current holder of Class B stock is listed as having no Class B stock following the offering. This includes CEO Mark Pincus, and several venture capital firms. In fact, four of the VC firms — Foundry Group, Union Square Ventures, Avalon Ventures and Institutional Venture Partners — hold only Class B stock.  So are they selling out completely?

No, according to a source familiar with the matter. It is true that the only sellers will be Class B holders, but the filing is “incomplete.” In other words, that empty post-offering column for Class B shares won’t actually be empty when all is said and done. Not quite sure why Zynga’s lawyers left it blank, except perhaps because the company hasn’t yet worked out who will sell what.

That said, the only significant institutional backers not thinking about selling any shares are Class A holders Morgan Stanley Mutual Funds, JPMorgan Chase and Capital Research Global Investors.

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