HuffPo moves the ball forward again:

Mitt Romney has not released his full tax records from 2010, including key documentation connected to his Swiss bank account.

...Romney released his 2010 tax return in January of this year, a document that first informed voters about the existence of his Swiss bank account and financial activities in Bermuda and the Cayman Islands. But people who own foreign bank accounts are required to file a separate document with the IRS that provides additional details on such overseas bank holdings, and Romney has not released that form to the public.

The Romney campaign did not respond to HuffPost’s request to view the document.

Swiss bank accounts don’t in and of themselves help taxpayers reduce their burden when used legally. But as HuffPo explains, this matters for another reason:

By serving as a curtain between Romney’s U.S. accounts and his foreign holdings, Romney’s Swiss account could shield many financial activities from American scrutiny. Hypothetically, any politically unpopular investments, clever and complex asset sales designed to lower Romney’s tax bills or other activities would be far more difficult to decipher.

The unreleased form would make it easier to tell whether Romney did engage in any of this, HuffPo notes.

As far as I can tell, this goes directly to what tax experts told me yesterday: It’s likely that Romney has paid a far lower tax rate in multiple years, and that this could help explain why Romney won’t release his returns. If he paid nothing in any given year, of course, that would be even more politically toxic. But even a far lower rate would be hard to explain. And as the experts told me, the returns could also reveal how Romney reduced his rates — whether he relied on “aggressive sheltering,” which would be politically very problematic, or more standard techniques, which would still be hard to explain, depending on how much lower his rate ended up being.

I asked the Tax Policy Center’s Joseph Rosenberg about this, and he confirmed that today’s revelation is another piece in the broader pattern those experts talked about yesterday. “There could be assets abroad that wouldn’t necessarily be shown in the standard tax return, but would be on this other sort of disclosure form,” he said. “What are these accounts, and what is the purpose for having them?”

If HuffPo’s story is correct, we don’t even have a clear picture of 2010, the year Romney is holding up as proof of his transparency. And of course, having more info about 2010 could hint at the techniques he may have used in other years. But again, this is all speculative — until, that is, Romney releases his returns.