As regards the source of the tax return:
1. The marking "Client Copy" may be meaningful, although even if accurate it tells us very very little about the source of this leak. I'll explain that below. But any tax professional/IRS personnel realizes the stamp etc down there if left original could indict them. Whether IRS or preparer firm, you're gonna stamp it "Client Copy" if you don't want the trail leading back to you.
2. IRS as source. I think it highly unlikely if only because I don't believe the IRS is going to keep a hard copy of a tax return for so long. The IRS receives millions of tax return every year. Record retention/storage space has been a big problems for them for decades. Hang on to the hard copy of tax return whose statute of limitations expired approx 10 yrs ago? I don't see it.
3. Tax preparer as source. I'm disappointed the preparing firm's name is redacted. But I don't see a firm releasing it. Again if only because we also have storage space expense/difficulties. The industry standard for retention is 6 yrs. I can't see a firm keeping a copy beyond that. I think it unlikely.
4. Trump as source. Not much to say other than, welp, another day another conspiracy theory by the Left.
5. Others. Considering all the businesses he's active in, or was active in, I'd guess about a gazillion copies of Trump's tax return have been sent to bankers etc. I.e., a metric shit ton of banks, firm's and people have had copies of Trump's tax returns. It could have come from just about anywhere.
That copy seems to have been xeroxed a number of times. That indicates it passed through a number of hands. I don't see it as a copy of Trump's original (client) copy. A copy of that original would be much sharper/clearer.
Unless somebody fesses up I don't see us finding out the source.
Fern