These things are not really a detachable MAGAZINE, per se; they are a magazine EXTENSION which attaches to the bottom of the triggerguard after you remove the floorplate/spring/follower assembly.
Originally, they were issued with a little steel clip so you could take the guts out of your original rifle mag, clamp them together and stick them in a pocket while you were usng the extension. On the mag extension, there was a little holster welded to the side; this is where you put the floorplate depressor after you attached the mag extension to your rifle. The mag spring was partly compressed inside the thing, forcing the follower upward; you needed this so that the guts of the mag extension didn't just zoom off into space.
The way you used it was like this: you compressed the spring in the mag extension and ut the little retainer through 2 slots near the top edge of the thing to hold the extension floorplate and spring down in the extension body. You then removed the floorplate/spring/follower from your rifle, clipped it together and slid it into a pocket. NOW you could attach the extension to your rifle, slipping it on and locking it in place exactly the same as the issue floorplate/spring/follower assembly. At this point you could pull out the little clip, releasing the floorplate to slam upward, filling the internal magazine box as well as the extension. You then put the little retainer into its holster; it was held on by a bit of jack chain about an inch and a half long.
NOW you loaded your rifle from single rounds or from chargers, usually BOTH. The mag extensions were in 2 sizes, one which attached to the rifle to give you 15 rounds total capacity, the other giving you 25 rounds total. That is a LOT of spring, so, more than likely, you could load perhaps 20 rounds into the big one, using chargers, then follow up with the last 5 loaded singly. Now you closed the bolt of your rifle and you were ready for the next Canadian offensive.
These things are known to have had 2 manufacturers. The ones marked on their back with letters "B" over "N" are made by Bing of Nuremberg, a popular toy manufacturer. There also is a manufacturer who marked their product with a "J", but it doesn't seem that this manufacturer's name is known. BVut these are not illegal high-capacity magazines because (a) they are toys and (b) the actual capacity of the thing when removed from the rifle is ZERO rounds. You cannot use one to quick-load a week's worth of ammo into the rifle: it has to be loaded empty, then you spend half an hour filling it up.
These were experimented with on the Western Front after Jerry had had a dose or two of SMLE rapid-fire from the 10-round Lee mags. They increased the dry weight of the rifle by nearly a pound and increased the loaded weight by close to 2 pounds..... on a rifle that already weighed close to 10 pounds with its leather sling and Seitengewehr 98 bayonet (with the 20-1/2" blade) attached.
They WORKED but they were not very practical from the standpoint of bulk and weight alone. Add to that the fact that the attached mag extension left a thin slit open at the juncture of mag extension and rifle body (a wonderful place for mud to enter) and you have a failed experiment. Quite a number were issued in 1916 but most were withdrawn later on, in 1917. My old friend Pte. Jack Snow escaped from a Jerry POW camp near Heilsburg in Ostpreussen, not far from Konigsberg, in late May or early June of 1918 and headed East with 2 other guys. They were trying to get to the Russian lines so they could be sent across Siberia to Canada, then to England and back into the war. They didn't know that there had been a couple of revolutions in Russia and that they were walking right into the middle of the Russian Civil War. They were within sight of Smolensk when they finally had seen enough and gave themselves up to the Germans, who were pulling back from Russia for the last 'big push' on the Western Front. Jack said that several of his guards still had these magazine extensions on their rifles, although they seemed to have been recalled in the West before he was "killed in action" (actually wounded and MIA, but that all got sorted out after the war) at Monchy-le-Preuex in April of 1917. That they still wewre being used in the East would indicate that they still were useful in the battle conditions pertaining on that Front.
BTW, Jack also said that, "Fritz kept us drunk all the way back to Heilsburg, then dropped us off at the clearing station. We got 3 weeks solotary on bread and water for escaping. Fritz kept his word on the bread and water but we were three to a cell; a lot of guys weere escaping then."
I have an original Bing mag extension (the big one) and it is a real pain in the butt to try to use. It ain't for sale, guys.
So there you have it. Hope this is of some help.