You are quite right. C5OE means the part was engineered in 1965 (C5) and paid for by the Fairlane Group (O). Might have been used for 2-3 years before a revision was needed.The casting numbers show the first year that part was made, but often the same part gets used for a few years. If they made a 2V intake casting in '65, and didn't change the casting (which would change the part number) for a few years, then you can see why a "C5__" part number might be original on a block that was cast in 1967. The block part number and casting date are on the lower passenger side rear part of the block, roughly where the starter overhangs it.
That does not mean it was installed in a Fairlane, it could have been in anything that had a 289 engine.