I've never seen a flapper that didn't have a spring. (Cadillac, Olds, Chevy).

Some are operated thermostatically like the Chevy while others are opened by the exhaust stream and closed by the spring.

I think you're probably missing parts. It probably had a weight on it that held it shut - maybe with the help of a spring - and it opens under exhaust stream pressure.

1950 Chevrolet 3100 (Ol' Roy)

1939 Packard Standard Eight Coupe (The Phantom) | 1956 Cadillac Coupe de Ville (The Bismarck) | 1964 Ford Galaxie 500 coupe |1966 Ford Mustang (Little Red) | 1969 Pontiac GTO (The Dishpit) I 1979 Ford F-100 | 1976 Ford F-150 (Big Red) | 1995 Ford F-150 (Newt)