The hole so noted in where the Sear Screw attaches to the lock plate on the inside. It holds the Sear in place. The "Sear" is a lever device that works between the trigger assembly to what is known as the "tumbler". Basically it is a vital part of the system that holds the hammer back at Half-Cock and Full Cock. If the Sear screw is missing the Sear probably is too. Without it the hammer wont function properly. The screw hole just above it that appears to be present is the Sear Spring Screw that holds the spring for the Sear in place.

If its just for a wall hanger it would be fine, it seems complete otherwise. If you want it fully functional I would recommend it be taken to a qualified gunsmith that has knowledge and experience with these old percussion locks. Replacement parts can be had but especially on these originials they typically require some specialized shaping and hand fitting so it works properly in that particular lock. If the Sear and Sear Screw are missing in the lock it might have other parts missing as well.

Frederick