I, and perhaps many others, have often heard that the in-line rifle is not a modern design. Yet, for many years I have attended one of the largest antique arms shows in the country, and I have never seen any type of muzzleloading rifle that even closely resembles having an in-line action. The closest I can find is some of the Civil War carbines. However they were designed to load from the breech, and used various types of cartridges.
Fladerman's "Guide to Antique American Arms", perhaps the most read publication in the antique arms field, shows nothing that remotely resembles an in-line rifle. You'll search in vain in other books as well.
So, I can only conclude that the in-line rifle is strictly modern, unless someone here has facts that prove otherwise.