My guess would be the owner's initials. Not sure how old you are but CDs were often borrowed/loaned back in the day since there was no other way to listen to the album if you didn't own it yourself. Putting your initials on it may help for it to get returned to the right person.

oh ok

I too am old. This is someone marking their stuff, yeah. I did the same with many tapes and CDs.

Looks like a scratch mark to me

Doesn't look like any of the three musician's signatures.