Terry Teachout opens up a review with this:
As an actor, Hal Holbrook has two real-life Marks to his credit, Felt and Twain.
That's pretty funny. I also note that Holbrook's "two Marks" are also strongly pseudonymous: Twain was a pseudonym for Samuel Clemens, while Felt is famous for his pseudonym, Deep Throat.
OK, I guess that's not as funny as Terry's line. More of an amusing correlation. Or something.