Plymouth Fury Hardened Valve Seats for unleaded gas date?

Hello,

Anybody happen to know the exact year+month that Plymouth started rolling out Furys with hardened valve seats so that they could take unleaded gas?
Had a search online and cant find anything more accurate than 'early 70s'.

Thanks

Jon

The 1971 model year (starting production in the late summer of 1970) and later used the hardened seats, as per government regulation at the time.