Tinker,
I finally got to take the pictures of the sights you had asked for. And since it is such beautiful, I took another picture of the buttplate as well as the sideplate which has the additional signature of Anton Mulacz on it.

What if the 18°8 at the firing proof would indicate the caliber, meaning 18,8mm? This would very closely fit to the diameter the muzzle has.

Fuhrmann,
many thanks for this PDF file. Since it anyway fits quite precisely to the 12 gauge shell, I suppose it must be something very close. It might even be listed in your document.

Would anyone dare to make a guess on which market value this gun has? It is very tough to get reliable opinions since not only there are hardly any references to find, but additionally the few to be found are distinctively different from this one.