Probably a casting set-up error. The "8s" should have been "9s". Is you car a late '68 build or a '69? No QA ISO 9000 back then.
Stamping and printing errors in coins, bills and stamps are worth a lot. Maybe in intake manifolds too.

Early '68s used a 9790140 intake manifold.
Late '68s used the 9794234.

Here's a similar discussion from 2013 where one had the numbers, 3784234: 68 400 4 speed intake casting number

And a PY discussion where a guy had a 8784234: Cast Iron Intake 8784234