I rarely ask questions that don't give you the answer you need to solve your problem. If you peek in the hole in the frame, you will see the end of that bolt. If you can see the end of it, that also means if you flip it around, you will see the head of it and the head of it is easily reached and torqued with an extension on your torque wrench and the appropriate socket. If you don't want to find a nut to replace the flag nut, vise grip on the mount will stop it from turning while you torque the other side. That bolt also got in there somehow, that means it will get out of there somehow.