"very rare prototype enfield ww1 spare mag. Carrier for smle rifle" (?)

[Ridolpho]

Not Lee Enfield, of course, but it's progenitor the Remington Lee was advertised as being used with multiple magazines and, according to Myszkowski ("The Remington-Lee Rifle") was used in this manner in combat. As per the LoC, the concept was initially somewhat similar in British use but rapidly "evolved" to easily removable, but not to be removed, magazine, initially with no facility for stripper clip use.

Ridolpho