LT,
You have a .58 cal Huntsman with a screw in breech plug, that has had the OEM sights replaced. They made a wrench for that breech plug, but unless you can find one, you will have to make one for it. The breech plug might be siezed up, but first I would make dure there is not a load in it. Once you determine that it is indeed unloaded, then you can safetly work on the breech plug. If you have a nipple wrench, you could try unscrewing that, you might get lucky and the whole works will come out. If the nipple turns out on it's own, I would plan on replacing it anyway's as it looks like it is worn from the pin.
The breech plug wrench will have a T-Handle, with two prongs that fit into the two holes, one on each side of the nipple. You may need to use some type of penetrating oil if the threads on the breech plug are locked up, if you want to remove it.