Is this a Fulton box lock or side lock? If it is a side lock, the rotary bolt holds it in place. There is a spring below it. The notch faces towards the breech and when the top lever is released by the closing of the gun, the rotary bolt which is attached to the top lever by a coupler which will move the trip downward causing the barrel to close on the receiver and bring the top lever to center.
Make sure you put the pin at the end of the coupler in the rotary bolt correctly or else you will not get the top lever to move smoothly and will jam.
