The production year for the '68s was about 8/67 to 6/68 so yours could be considered a later '68 build.
The "The Definitive Firebird & Trans Am Guide 1967-1969" by Rocky Rotella has a section about the '68 intake manifolds. It mentions, "The earliest date for the 9794234 manifold has been March 25, 1968 (C258)". It goes on the imply that that date may be the earliest 9794234 found. Rotella says that the drawings for the 9794234 indicate that it was designed in Feb. 1968.
