Is This a Civil War Era Austrian Lorenz?

[Patrick Chadwick]

I measured the bore diameter again and it measures 19.19mm inside diameter, and 23.88 outside diameter.

So my 23.7mm, scaled from your photo, was a pretty good estimate!

The thing that perplexes me the most, is that if the barrel was originally manufactured to be used on a fowling piece, why the bayonet lug and front and rear sights?

This is getting very speculative (but it's fun!): It may, of course, be a barrel recycled from a musket. Take out the barrel and see if you can find a trace of an ignition hole for a flintlock - if there was, it would have been plugged or welded shut. It is also conceivable that, if there was a flintlock ignition hole which was badly eroded, the person doing the conversion converter simply cut off the breech end of the barrel and recut (lengthened) the screw thread for the breech plug.