Lorne, on that IRS letter dated May 13, 2015, they thank you for the letter you send them . they say they received it on April 10, 2015, but they don’t tell you the Date and title of the letter you sent them. If you have sent more than one letter , you don’t know what letter they are referring to. Ask for copy of your letter.
Do an affidavit, I understand IRS is a commercial collection agency. Under the commercial procedure disputes in commerce are by sworn affidavit as true, correct and complete.
An unrebutted affidavit becomes judgment.
I am sure they have never provide you with an affidavit that provides validity to your assessment. And one doesn’t exist according to your belief.
Fact: you send them copies of you pay checks as evidence you don’t pay a user fee in accordance with 12USC sec. 411.
They haven’t disputed that fact yet.
They say you have 60 days to answer them back. You still have time.
Look at “instructions: 0.5 instructions: 0.5 commercial law and the uniform commercial code (UCC).
It’s under famguardian.org.

in one of their other letter they state they will send you a "statutory notice of Deficiency".
I believe Statutory law in an other name for Admiralty law.