The magazine on this rifle is, as has been shown, from a ZH-29.
It is nothing at all like the First World War extension magazines for the Gew 98. With the Gew 98 rifle, the magazine box itself is built inside the frame of the rifle. What the extension was, was a box equipped with its own spring and follower. This box had a lip on the top. You REMOVED the spring and follower, along with the floorplate, from your Gew 98 rifle, clipped them together with the little spring clip that came as part of the kit, and stuck it in your pocket.
The magazine box that you now fitted slipped on the rifle, replacing the original floorplate. You with drew a small crosskey and the follower slammed upward to the feed rails, which are part of the frame. Now you opened the bolt and stripped in either 15 or 25 rounds. The so-called "20-round magazine" was really a 20-round EXTENSION of the existing 5-round magazine within the rifle. The so-called "10-round magazine" was in fact a 10-round EXTENSION of the rifle's built-in 5-round magazine.
OUT of the rifle, either extension was not suitable for carrying ammunition around in: no lips at the top to retain the cartridges, AND a very long, rather strong spring. But they workedreally well when on a rifle.
An old friend of mine escaped from the Stalag in Germany in 1918 and spent 5 months wandering through the middle of the Russian Civil War. When he gave up to the retreating Germans and was escorted (drunk, courtesy of Fritz) all the way back to heilsburg, he recalled seeing some of the extended magazines on the German rifles at that time. According to what he could remember, they were already being withdrawn from german service in the Spring of 1917, when he was on the Western front.
The German extension magazines were made by Bing of Nuremberg, a toy manufacturer who marked their magazines with a B over N and a horizontal line separating them, and by an unknown manufacturer who marked theirs with a J.
Hope this helps.