These lenses are notorious for having a glued in internal baffle come loose inside the lens between the mount and rear element. Its just a thin shim shaped like a doughnut that is larger diameter than can easily be accessed from the rear mount tube/barrel. Originally it was glued from inside before the rear element was installed.....THERE IS NO WAY TO RE-GLUE IT !
You have two choices......leave it alone and the lens is useless and cannot be used.....OR.....reach inside , bend it , and pull it out of the rear with a pair of needle nosed pliers after removing the mount.
If You look for them you will occasionally find a copy with this issue from a Japan seller on E-Bay listed as PARTS ONLY/DEFECTIVE...…..I have had one do this but I traded a $25 brass tumbler for it and I cant see any difference between my 3 copies at all......I have less than $100 total to acquire all three of mine and two had this minor issue.
Its a good way to get one of these for cheap if you can find one.
I was going to post a link for one currently on Ebay but it was snatched up a couple days ago !