You may have seen that on the "technical updates" page of a later rev of JM's '69 book. I happened to examine a very original early production Norwood Z/28 that had a 5460346 master stamped US and passed the info along. I have seen earlier cars with 309s so maybe this was a case of Chevy using up some inventory.
The car in question was much earlier than yours, 09D. An 01C car would have the 309.