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The letter is just a pattern identifier. See: http://www.castironcollector.com/numbers.php for more info. One with the same characteristics as yours is shown in the blue book valued at $35.
This griddle apparently went through some changes as seen in the form of ghosts on some of them. The trademark was at one point in the center, with just "ERIE", and there were no reinforcement bars. The logo was moved to the bottom when the bars were added. Also interesting about the ghost marks is the centered logo is seen oriented 90° from the normal.