I have a nearly identical one. Take the pin out of the foreend wood by pressing the detent behind the button. Note which side it entered from. Put a soft lead bullet tip into the hole in the mag, there will be 2 of them, but only one will be spring-loaded. Push the spring down and swing the mag plate to one side and lift it out and remove it. Then switch the lever beside the trigger guard 180 degrees, (to the left, from memory). Put a small amount of thumb pressure on the barrel to lift it from the foreend and also, you might put some pressure on the inside of the mag to push it away from the stock. Make sure it can fall on something soft like a towel if should pop out. Go steady the first time unless you have a spare stock! There could be rust and etc. holding it tight.

This is the take-down model and the entire action should come out of the stock. The rear of the action has a small notch, by the way.

If you do this, you'll find those numbers, something like 346.28, which would be the 346th firearm proofed in Vienna in 1928. That's the proof-office number, not to be confused with the serial number which is visible without moving the stock, for instance, on your bolt-handle.

There's a detailed article on this Mannlicher take-down 1910 model in 9,5x57 M.-Sch. in the latest issue of New Zealand Guns and Hunting magazine. Issue #126, Sept/Oct 2011. Modesty prevents me from mentioning who wrote it.