Thurnau is the name of the City in Germany
The Bbl address may be the retailer and not the gun maker.
It looks like the name is T. A. Haein
The AE joined to appear as one letter as they often did.
The letters were Silver inlayed but some of the inlay has fallen out.
The A. after the name and before the City is my poor guess,, probably for 'Aus' (from)
Looks like a conversion from Flint to Percussion. The lockplate shows the small support feature on the bottom edge of the plate for the frizzen spring screw and a plugged hole right above it.
The Percussion locks usually were made using only one side lock screw. Flint locks using two. This lock plate has the flint second lock plate screw at the extreme front end of the plate.
..and the whole gun could just have been made up of usable parts from other guns at some point in the past. Nothing usable ever got trashed untill we got to the latter part of the 20th Century.
Nice looking piece and certainly worth the money!
Put a rod down the bbl and check the length against the outside of the bbl to make sure it isn't still loaded.