BATF opinion letter on 80% receivers
Here. I note that the letter is trimmed to only include "Sample 1," presumably because that's what they're selling, and the other two samples may have been found to be "firearms."
Under the Administrative Procedure Act, 5 U.S.C. 552, documents and rulings such as this are supposed to be public, not just reflected in a letter to one person. Any general doctrines of, in this case, how much finishing it takes to make an item legally a "firearm" should be published in the Federal Register and then in the Code of Federal Regulations. Specific rulings (rubbing your belly and saying this is or isn't a firearm) should be in the agency reading room (these days an online one) so that the public can see and be guided.
Hat tip to Dave in Colorado