I think I have it nailed.
The only logical guy this could have been would have been Johann Keck.
Keck served in the 6th Company from 1914 till the end of the war. He therefore qualified for the ring. He is however not on the Ring Roll. He should be there as he fulfills all criteria.
A "Heck" is on the roll, but not listed in the 6th company. In fact, I cannot find any Leib regiment guy named Heck who qualified, and I went through all the Heck's in the Bavarian Army.
The only stumbling block then is Keck is a Gefreiter... but seeing all the errors on the Roll, that is more of a questionmark than a "no!"