Magazine cutoffs were a staple of some 19th/20th Century military rifles. They were to be used in single shot mode, with the magazine in reserve, till some wise officer ordered them released, and then to be used as repeaters.
This modification apparently is not for rifles alone, as Webley automatic pistols have this feature as well, no doubt mandated by some government agency. The system works with the magazine, which has an extra locking hole, where the magazine is not inserted all the way and locked in the second hole, keeping the bullets held in reserve and out of battery. The slide doesn't use the magazine to lock open after firing, and the shooter has but to drop another round in that big .455 hole and continue shooting. If repeating operation is desired, the catch is pushed and the magazine shoved home.
The feature does seem odd in a pistol, but it is simple and it works. The commercial pistols seem to have this cutoff also. The original idea of a magazine cutoff was an officer controlling the behavior of his troop of men. In the British Army, there were few men who weren't officers and carried pistols. Very confusing. At the time cutoffs were in vogue, the idea had some merit, as the British Army was usually a long way from home, really outnumbered, and every cartridge had to be carted the whole way on someone's back.
My friend Roy Shadbolt kindly sent these pictures of his .455 Webley pistol showing the magazine cutoff details.