That is a false muzzle. it is a separate piece that is pinned in line with the bore and then rifled along with the barrel. The patched ball can be started down the bore and seated and then the false muzzle is removed before shooting. That way the muzzle is perfectly square and all the wear from loading and cleaning is to the false muzzle.
And yes they make a left handed model
There is actually a dove tail cut under the barrel lengthwise for adding stabilizing weights, if desired.
The purpose of the pin sticking up, is so when sighting down the barrel, it reminds the shooter to remove it before firing, they have been known to fly down range.