Fireplaces, Heating Stoves, Flues and Chimneys - throat damper bolt broke

I have an 35 year old brick and mortar fireplace with a screw type throat damper. It worked great, but the bolt that held the rear part of the screw mechanism broke and the mechanism is now busted. I think I can remove the parts of the screw actuator, but am unsure about it. Has anyone out there fixed such a device? It looks like a set screw holds the shaft to the outside twist handle, and a cotter pin holds the rear end of the mechanism--that's where the bolt broke so it came away from the back of the damper box. I hope the damper/blade is removable to allow access the the other side of the bolt hole because the blade seems to cover up the hole where a a replacement bolt would go. I hope it is a bolt and wasn't welded or something. Any help???