The following information on the rise of the Mauser clip-loader comes from Chapter 13 of Mauser Rifles and Pistols by W. H. B. Smith. Mauser Rifles and Pistols is also available to purchase in print.
Shortly after the introduction of the Model 1888 rifle into Germany, Belgium considered adopting a similar type of multiple (or packet) loading arm. However, after careful consideration and tests of loading systems, it was decided that a more desirable form than the Mannlicher would be one in which the cartridges could be inserted singly to reload the magazine when time or opportunity permitted; in which the clip would not constitute a necessary part of the magazine mechanism, since rusting or deformation of the clip could result in jams; and in which the desirable factor of single-motion magazine loading could be retained.
The Belgian Army therefore requested manufacturers of small arms in all countries to submit rifles into which the cartridges could be speedily inserted in groups as in the Mannlicher, but without the drawbacks encountered with the packet system of loading where the arm could not be singly reloaded or the magazine held in reserve during battlefield use. It was stipulated that it was indispensable to the complete reliability of the arm that the magazine should be such as to permit refilling with single cartridges at any time. This principle was upheld in battle experience until the introduction of the U.S. M.1 Rifle in 1936, by which time we had so perfected our supply system that a packet loader was feasible.
From this request of the Belgian Government developed the next Mauser rifle, one based on an invention of Mauser’s patented under specification No. 12689 in England in the year 1888. This device was the now familiar cartridge clip upon which five cartridges are mounted so they can be stripped down into the magazine without inserting the clip itself. This system permits reloading with individual cartridges whenever the magazine is partly depleted and opportunity permits. This strip-in clip, by speeding up magazine loading without danger of jamming, completely revolutionized the development of military rifles.
Ironically enough, the first official tests of the new Mauser loading system were held in England; and the British War Office passed up an opportunity to be the first in the field with this new form of speed loading.