While the rumor exists of (very) early production examples lacking a VIN sequence stamping, I have never seen a verified example to prove it and plenty of early built cars that did have it. It's much more likely that your engine either has a VIN sequence stamping and you can't find it, or that the engine's been replaced at some point. Most 68 Hemi cars built prior to Jan 68 would have had either a 65 or a 66 casting block, these would have also been stamped with plant and assembly date info on the lower pan rail bell housing extension on the passenger side of the block. What is your cars scheduled production date?