The details underlying this question are on the Company of Military Historians forum, but the only question I have has to do with USN ship armament ca. 1875.
"Did any USN ships carry broadside-type 15-inch guns ca. 1875."
Why do I ask? The 50-lb., 1/10 scale model features a Marsilly carriage, which is NOT used in a monitor, and prior to studying this model, I assumed the only shipboard use for the XV-inch shell guns was aboard Monitor-type vessels. The existance of this antique model, probably government-made or contracted, means the Navy must have intended this configuration for use as a broadside gun for non-monitor-type ships. The Navy had exactly 20 of the "pattern of 1870" XV-inch guns cast at Ft. Pitt foundry in 1871-2.
Again details are on the other forum but the large model cannon almost certainly represents the Navy Bureau of Ordnance XV-inch Shell Gun, (pattern of 1870.) I put "pattern of 1870" in parens because I don't think the Navy ever used that terminology, but I have to separate the unique pattern from previous versions.
Olmstead did something similar, "THE BIG GUNS" has a good writeup on the various versions of the XV-inch shell gun. Unfortunately on pp. 92-93 the drawings are repeated on both pages, so the one that should have appeared on pp. 92, the "short" early Dahlgren XV-inch, ca. 1862, isn't in the book at all.
If anyone is really "into" this and has access to a drawing of the "pattern of 1870," I'd love to see it.