As someone who has used side-saddle shotgun shell holders on my 870s and Mossberg 500s for decades, the Metcalf Device was always pretty interesting to me.
Introduced by Lieutenant Henry Metcalf and patented in 1876, these 8-shot boxes were to be carried on an infantryman’s belt and then mounted on a Springfield breechloader, or similar. Shown below on a Martini.
Only about 1,000 were made for Springfield 1870/73 rifles, and they were soon discarded as impractical.
RIAC has not one but TWO of these oddballs up for grabs, and has an excellent video on it, below.