Sorry to show up so late to this post, but I've only just joined and my searches are just now turning these things up.
Nearly 1,244,000 P14's were produced by the 3 U.S. factories and issued to Britishforces, seeing limited service on the Western Front before diminished need for new rifles saw them used for support units and passed on to commonwealth forces. The initial prototypes of the P13, with aperture sight, were designed in 1910-1911, and apertures were fitted experimentally to various Enfields from 1900-on, as a result of Boer War experiences.