Guess you're right. My F53 uses many of the same parts as an F550. pretty sure of that because I've already bought and put some F550 parts on it. Ford doesn't classify their trucks medium duty until the F650's. (I looked it up.) Didn't know that where Argosy obviously did, but it has affected my parts sourcing up 'til now not one bit. I had an idea a 19,000 pound vehicle might be on the the lower end of medium classification. Evidently not. Glad to have learned something today though.