I’ll admit to knowing little about Gunther (though his book is sitting on my shelf waiting to be read at some point).
The skeptical side of me reads this and can’t help but wonder if he didn’t send this to give his forthcoming book an heir of legitimacy.
If Gunther was contacted by the FBI, it doesn’t seem that hard to tell them a BS story without them catching him in a lie. Consistency is the key. As a journalist, he may have gotten a lot of calls from payphones back in the day. How could the FBI look at his phone records and know what was what?
I’d love to find out it’s a legit clue.