You could have said that right at the start!
And I thought you were unhappy with the rifle!
I wish I had that kind of problem! You have acquired a free(?) rifle, equipped as a sniper, that shoots excellently. The stock disk is neither an issue nor a problem. It is a triviality. If it bothers you, get another from an arms fair. But Dana Jones in "The Crown Jewels" writes that the disks were often removed, and a replacement will never be original to that rifle - and will not improve the accuracy either. Remove the incorrect disk - which is a fine example of how trying to recover a so-called "correct" configuration can lead to a falsification that actually has a negative effect - and enjoy your good luck!
"Under 2 inches at 200 yards" is what matters, not a bit of often-faked brass.