This might sound crazy, but I believe the small studded shell is Japanese. I know of the existence of only 3 of these, and they are all from either the Japanese Boshin War 1868-69 or the Southwestern War of 1877. They are all 8.7cm in diameter and are for the French 6-Pounder La Hitte howitzer, which were exported to Japan in the 1860s. Interestingly, only this caliber has these style studs; the 4-pounder La hitte shells (8.4 cm) recovered in Japan look identical to your common French La hitte shell, because they were the exact same ones that France was using. These 8.7 cm shells with the thick copper studs are thought to be Japanese-manufactured for French-imported guns. Does yours happen to be 8.7 cm in diameter?
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I'm afraid I cannot say much about the 6.6" shell, but I would like to emphasise that the AI descriptions are very bad at identifying things like this, as they make up information without having real sources. I wouldn't trust anything other than period artillery docs, manuals, or treatises, or sources that cite period documents. That said, do you know where I can buy a copy of David Ibbetson’s book? Sounds like the perfect book for my research.