There was another case of an Offenhauser powered entry in an F1 race. Roger Ward drove a Kurtis-Kraft Offenhauser midget racer in the first USGP, held at Sebring in 1959.
Ward won the Indy 500 that year. He drove his Frank Kurtis built Offy midget to a victory against the world’s top sports cars, including a Maerati 250F 1 car, at Lime Rock, Connecticut. This gave him the idea to run the car in the USGP. The longer, faster circuit didn't suit the little car. Ward qualified the car a full 43 sec./lap behind Moss's pole-sitting Cooper.
After the race, Ward talked to John Cooper about bringing his cars to Indy. Cooper agreed and changed Indy forever the next year.
Ward won another 500 in 1962. He had one more F1 start, in a Lotus-BRM at Watkins Glen in 1963.
The pix is Ward passing under the MG bridge near Sunset Bend at Sebring.