I have that button on my '74 Charger...seems like it was something to do with a seatbelt/ignition reset button...don't remember for sure though...definately was not a starter button.

1978 Round Up-1977 Warlock-1977 Powerwagon Adven.- 1976 Blue "Macho"-1974 Charger-1968 Fury III -1966 Newport *