I don't think the FS 8553 PT-10 is for early 273 engines. I did a product search on Rock Auto, and this is what I came up with:
CHRYSLER CORDOBA 1978
CHRYSLER LEBARON 1978
CHRYSLER TOWN & COUNTRY 1978
DODGE ASPEN 1978
DODGE B100 1979-1980
DODGE B150 1981
DODGE B200 1979-1980
DODGE B250 1981
DODGE B300 1979-1980
DODGE B350 1981
DODGE CB300 1979-1980
DODGE CHALLENGER 1970-1973
DODGE CHARGER 1970-1978
DODGE CORONET 1970-1973
DODGE D100 PICKUP 1979
DODGE D150 PICKUP 1979-1981
DODGE D200 PICKUP 1979-1980
DODGE D250 PICKUP 1981
DODGE D300 PICKUP 1980
DODGE D350 PICKUP 1981
DODGE DART 1968-1973
DODGE DIPLOMAT 1978
DODGE MAGNUM 1978
DODGE MONACO 1978
DODGE POLARA 1972-1973
DODGE RAMCHARGER 1979-1981
DODGE RD200 1979-1980
DODGE W150 PICKUP 1979-1981
DODGE W200 PICKUP 1979-1980
DODGE W250 PICKUP 1981
PLYMOUTH BARRACUDA 1968-1973
PLYMOUTH BELVEDERE 1971
PLYMOUTH CARAVELLE 1978
PLYMOUTH CUDA 1970-1973
PLYMOUTH DUSTER 1970-1973
PLYMOUTH FURY 1978
PLYMOUTH PB100 1979
PLYMOUTH PB150 1981
PLYMOUTH PB200 1979-1980
PLYMOUTH PB250 1981
PLYMOUTH PB350 1981
PLYMOUTH ROADRUNNER 1968-1973
PLYMOUTH SATELLITE 1968-1973
PLYMOUTH TRAILDUSTER 1979-1981
PLYMOUTH VALIANT 1968-1973
PLYMOUTH VOLARE 1978
So, the earliest shown is 1968 Dart, Valiant, Roadrunner and Satellite.