The part of the Ruger carbine on which the end of the screwdriver is lying is called the cartridge guide plate. If it gets loose, you will experience feeding problems. Now is a great time to check it out, and make sure it's nice and tight. It's held in place by two screws, which can be seen on top of the receiver. Keep them tight.
Don't be alarmed if your rifle doesn't have this part. A newer Ruger manual from 1979 doesn't show it.