By Dan Primack
September 13, 2013

FORTUNE — A company’s stock is only worth the price at which it trades. And for Twitter, that price right now would be $31 per share.

Fortune has learned that broker-dealer Felix Investments today is offering its clients access to Twitter stock at that price, which would work out to a company valuation of approximately $15.5 billion.

To be sure, it is only one price point. But there may not be any others right now.

Sources say that Twitter froze its cap table around the time it submitted its confidential S-1 filing to the SEC, similar to Facebook’s (FB) pre-IPO strategy last year. Moreover, word is that Twitter isn’t even processing trades for existing holders right now (it’s always been stingy with its rights of first refusal).

RELATED: Twitter IPO will come sooner than you think

Felix is getting around this by not offering clients actual shares in Twitter, but limited partnership interests in a special purpose vehicle that already holds Twitter stock. Moreover, the shares were purchased early enough that they are not subject to restrictions. Thus no change to the cap table, nor any need to get Twitter’s permission.

Before Twitter began buttoning everything down, the rule was that employees hired between 2009 and 2011 could sell up to 20% of their options grant. Earlier employees could sell more than 20%, but not if they remained with the company. Those hired since 2012 were given restricted stock units (RSUs), which cannot be sold until a liquidity event (which looks to be the IPO).

A Twitter spokesperson declined to comment on the cap table freeze or trading restrictions.

It also is important to stress that private trading prices are not necessarily guides for where a company will price its IPO. For example, Groupon’s (GRPN) valuation climbed to $18 billion in private market trades, but only got $12.65 billion at IPO. Facebook also traded higher pre-IPO, while LinkedIn (LNKD) traded well below its eventual IPO price.

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