hello
have an answer from Josef Mötz "The weapon is certainly a unique piece by a gunsmith."
he had not seen anything like this before and thats means something. in the interwar years the austrian army use the Erma EL24 .22 lr conversion kit. https://www.youtube.com/watch?v=ydPVasq2t_Y&t=12s

before the war the old K.u.K. army was using the "Kapselschießeinrichtung" ( very hard to translate)
for short range and also indoor shooting practise.

and hard to find pics also, it was an insert, lathe turned from solid brass with a propelling charge looking like a big primer ( the Kapsel) and a 8 mm lead bullet bullet with a hollow base you set on the tip of the insert with a special pliers.

http://old.municion.org/8mann/8mann.htm

"Reusable reduced shot cartridge for training. Instead of a piston, a small 4.5x9 mm HP adaptor blank cartridge is inserted and a lead bullet of very small size and weight is manually inserted. It can be used in the ordinary gun without any modification. PHOTO: TONY"

your rifle is from the technical point of view half way between the old kapselschießeinrichtung and the newer .22 conversion kit.