Since he won't release his tax returns, and does admit they'd be politically damaging, all we can do is guess.
This is one in a series of speculative diaries wondering what could possibly be worth the price he is paying by remaining secretive.
Today's idea: he's formed a lot of partnerships over the years. And since his son Tagg was willing to take on partners still under a cloud from Sir Alan Stanford's ponzi scheme, I will bet $10,000 (Monopoly money) that Willard is not choosy about his partners. If there's money to be made, Romney will be there to make it.
So ... drug lords? Mafia figures? Porn stars? Saudi oil princes? Who knows who Willard has done business with so long as there was a buck to be made, or a tax shelter to be found.