I don't know about those particular ones, but the idea works OK. There was a 25 round mag made for the Mauser in WWI and a 20 round for the Model 1903 Springfield in the same period of disagreement.
Both work the same way, by replacing the rifle floorplate with a cut out one which has the magazine, with spring and follower, permanently attached. So they are not detachable magazines in the normal sense and cannot be kept loaded off the rifle. They are loaded from the top with the standard chargers (stripper clips).
HTH
Jim
