I just got a rack to mount on the rear of my bike frame, but I'm puzzled at which mounting holes to use. On the right side of the bike there's an obvious screw hole for mounting, but the corresponding spot on the left side has something in the screw hole so I can't use it. Hopefully you can see the silver obstruction in this picture -- mounting hole would be just above and to the right of the axle.
The other screws are holding on the disc brake, so I don't think I want to mess with them.
The bike is a Trek 3500, the rack is a Blackburn Local Spring Clip.
Looking for advice on how to proceed with mounting this.