says out of a 73 Firebird.
https://www.kijiji.ca/v-classic-cars/chatham-kent/400-pontiac-engine/1305221300?enableSearchNavigationFlag=true
Says 77 right on the back of the block and weaker casting metal so not 1973.
I suppose the ad says in came out of a 1973 Firebird, which could be true, not necessarily saying a 1973 casting.
Ray White, Toronto ON
1973 LeMans 454 "Astro-Jet"
Built March 9, 1973 - Oshawa ON
1993 Corvette Convertible LT 1
Built January 10, 1993 - Bowling Green Kentucky
maybe it got a new engine in 1977 . perhaps several owners ago