My original guess was that it'd be the RPG sampler, or perhaps the Kabukiden demo or another Hyper Catalog disc... -something that either didn't have a case originally or wasn't a full game.
I once found a Hyper Catalog 4 in with an Emerald Dragon I bought online years ago and many years before that I bought two Emerald Dragon cases literally off the street which contained Dragon Slayer II, Tengai Makyou II, Ys III and a Chinese war sim.
But today I was surprised to find that the Kabukiden case came with
Tengai Makyou II and
Ys IV.
Which makes that my 4th TMII disc I believe, but only my second Ys IV disc.
There weren't any manuals or anthing else inside, just the case with it's original inserts and the two CDs.
I was immediately shocked at how nice the case looked. I bought Kabukiden when it first came out, nearly a decade and a half ago and my case is severly worn.