I took a look at my sporran, which is very similar to yours and found that while the piece is in deed threaded, it isn't a nut that is missing. It appears to me that the screw head itself is has broken off. One of the screws holding the cantle on my sporran has broken as well. The screws that hold the metal cantle are attached through the back of the sporran flap, and screw into the cantle from behind. If you look closely at the front of the cantle, you will see that the area where the screw would logically come through has been ground down.

I am not sure if this type of problem can be fixed without replacing the cantle all together.

I'll post some pics here in a minute..