While you are on the site, in your browser find and click "Add to Home Screen" A CGN beaver app icon will then be created to your phone that is directly link to the site.
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Same thread as here with photo montage in the post showing it mounted originally on an Enfield, and how it worked on a 1917 ShtLE (Short Lee-Enfield) No.1 MkIII* (Mfg by BSA - Birmingham Small Arms)
Yup, about as rare as hens teeth. I saw one in real life once...once. And it had a hole in it, I suspect from a round that was quicker than the swinging action of the flap.