flipping up the small extension attached to the bottom bolster. Would guess there is a small 'plunger' in that extension that moves when struck by the added rod on the hammer. The plunger would then strike the lower percussion cap.
The lower bolster screw would need to be a 'shoulder' type screw to allow that extension to swivel up when needed.
A really rare variant that is in great condition and happily the gun still resides within the maker's family. The OP's photos are excellent. Thanks for the posting. Are there any stampings on the barrel?
Several makers came up with individual "one-off" mechanisms to fire both barrels of fixed breech over under guns with only one back action lock. None were a commercial success, so they are rarely seen. In my 60+ years of collecting percussion over under guns, I've never seen one constructed with this specific design. Taterbug hit the function of the missing transfer piece pretty well. The most similar one is on a Nathan B.Tyler gun from Vienna,Ohio . On his the "thumb piece" mounted at the bottom of the lock is pushed up to connect the auxiliary striker on the hammer with the bottom nipple.
