Daryl,
The rifle has 2.950 Gram so less than 3kg. Barrel length is 66,5cm, so quite long.

Nitrox,
Since I am only a part time hunter myself and not an expert on vintage hunting methods and equipment I could only speculate on the purpose of it. Therefore I leave this to those who have greater knowledge with this than I do. The reason why I am so confident that it never left Austrian possession is however the origin it has - it came from a quite large and advanced collection which was sold in one package due to death of the original owner. He lived in Vienna and knowing how long he had it leaves very little time span for the rifle having been abroad. Therefore it is quite safe to assume it never left Austrian possession.

I have obtained a brass 12 Gauge shell yesterday. It fits quite as precise as a normal 12 gauge case. Having spoken with some gunsmiths locally most - independent from each other - suggested the 18°8 inscription of the firing proof refers to 18,8mm barrel diameter at a certain point. This makes sense, since it has with a caliper measured 18,76mm barrel diameter at the muzzle.
One gunsmith suggested the paradox rifling might had been tried to be removed to convert the gun to a shotgun, therefore only being visible atop at the muzzle. The same gunsmith also suggested it once might had come with a second rifled barrel which would be a reason to also carry a foldable peep sight. He however agreed that this is only speculation.

My question on the value was mainly based on the pure interest whether the price I paid was good, fair or too much. As I had mentioned, my knowledge on such guns is limited and therefore would had appreciated any input, doesn't matter if these represent personal opinion.