It starts to get very difficult to. These groups operate almost like intelligence organizations and keep a tight lid on things.
My guess is that these letters are either the product of The LawMedia Group
or Sam Simon's Amplify Public Affairs
, but it's not like these guys leave a clean paper trail, and given that K-Street disinformation is such a big business, there's likely other companies who do this stuff who are even more secretive.
That's not to say I'm not still looking.