Beautiful bass, but weren't all of the '78s in the rectangular cases with the copper-colored lining? I have three '78s (one of which is an 8M), and they all have the rectangular cases.
According to all the sources I have seen, you are correct. All I know is that the seller of this bass claimed that his father had bought it new. It's entirely possible that the case was replaced with a later one at one time or another. Since there was a good deal of "Player's Wear" on this bass, the case may have just worn out with use, and been replaced for that reason. The number codes on the neck of the bass date it as a 1978.