Pg 50 of June 30, 2008 10Q....
"Available-for-Sale Securities
The Company holds available-for-sale securities primarily for interest rate risk management and liquidity enhancement purposes. Accordingly, the portfolio is comprised primarily of highly-rated debt securities. The following table presents the amortized cost, unrealized gains, unrealized losses and fair value of available-for-sale securities as of the dates indicated. At June 30, 2008 and December 31, 2007 there were no securities classified as held to maturity."
They are not held primarily for sale to third parties
Of that $24B+ in Available for sale securities, $18B+ were MBS's whose returns i suspect were pledged to WMI.
If WAMU held more Participating Interests in MBS's these would have to be recorded in it's filings.
This is because there were no bankruptcy issues pre September 2008, so those assets would have to be accounted for.
I have not seen any additional revenue streams in filings that would indicate WAMU held additional mortgage interests.