I believe you have a variation of or an experimental Lynall Thomas shell, from sometime in the 1850s or early 1860s. It's not a 100% match, but as you can see in the drawing I've included, it has the same rounded head and the very unique tail end "knob." I've never seen the rounded knob thing on any other shell (and I've been studying blueprints and drawings of hundreds of European artillery shells of this era), so I am fairly certain it is a Thomas design. For more information on these and the source of the drawing, check this out:
www.google.com/books/edition/Rifled_Ordnance_A_practical_treatise_on/qy5YAAAAcAAJ?hl=
The 1864 edition of this book does not include anything on this type of shell, suggesting it might be pre-1864. As far as I know, Lynall Thomas shells were never adopted by the military; some were sent to the US for the American Civil war but they are distinctly different from the ones described in the 1858 version, which I linked above.
Very cool piece! I'm very jealous because I just love the experimental era of rifled artillery in the 1850s!
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