I received one very helpful input via email, from a knowledgeable person:

"...consider the authority on the subject of French naval muzzle-loading artillery to be L. Denoix and J.-N. Muracciole, "Histoire de l'Artillerie de la Marine de ses origines a 1870." Although their line drawing illustrations do not show swivel guns, this piece falls closely into the designs of 1700-1750 for the larger guns then cast of iron."

I don't have the book mentioned, but as soon as I find one, I'll buy it if affordable, or photograph all the pages if it isn't.