He realized that the coin was similar in size to a five dollar gold piece and the “V” on the back lacked any mention of “cents”. He also guessed correctly that a sales clerk handed a gold plated version of the nickel would assume it was really the five dollar gold piece.
The brilliant part of his plan? When he was finally caught he successfully used the defense that, because he was deaf and dumb, he was unable to correct the clerks mistake in giving him back to much change.
How well did Mr. Tatum’s plan succeed? When someone figures out they’ve been conned with a line of BS, we still use the defense, “It’s ok, I was only joshing you!”