Remember this, it's important: 24 Kugeln (balls).
I'm actually a bit stunned at the moment. Never in my life did I expect to find such a document. It just didn't seem possible. But, here it is.
I love the old-German letters.
This will take some real time and effort to get properly translated and annotated. It's an amazing document.
Natale de Beroaldo Bianchini was the top honcho at the Austrian armory and in 1820 he published this book that describes all the Austrian Army arms and sabers, down to the level of steel manufacturing, screws, stocks, etc, etc, etc. He was the person responsible for Austrian Military arms manufacturing. And... under Special Weapons... he describes the "famous" Austrian Military Air Gun made by a "certain" Girandoni. It's all here: Barrel length, weight, etc, etc. The caliber, always one of my favorite topics.... 5 lines.
But, the most important specification, and this guy personally shot a Girandoni! - 24 Kugeln: the number of balls that the magazine held. So, unless your Girandoni-style repeating air gun holds 24 Kugeln, it can't possibly be an original Austrian Military Air Rifle.
Natale de Beroaldo Bianchini gives more than enough information to absolutely identify an original Girandoni.