It is obvious from the position of the nipple, that the drum is either a replacement with a different orientation than the original, or has been overtightened beyond its index with the hammer. In either case I would not continue shooting it until it is disassembled and repaired.
One of the most common misconceptions concerning shooting antique guns is, that as long as the metal parts show no signs of failure, you are good to go. But, the wood has been drying out over the hundred plus years the gun has been in existence as well, and is fragile. The sharp jolt produce by the firing of such a weapon can fracture the stock. This is especially true of a gun with a back action lock like yours.
Although a gun may be considered not particularly valuable today, it can become quite valuable in the future. Guns that had no known history, were unsigned by their builders, or were simply a cheap gun in their time, have gained value as new information becomes available.
Remember you are not the owner, you are the caretaker, and you should do your best to preserve this piece of history for the next generation.
Hungry Horse