Boom J you are correct; it's a City Class Casemated Gunboat. The Monitor was the "Cheeze Box on a Raft" first made in Greenpoint, Long Island, NY with the two 11 Inch Dahlgren Shell Guns. Double D, all the research I did on this topic is condensed pretty much into a couple paragraphs from that previous post and is now copied to this post and here it is:
Re: M-1841 42-Pounder Army Rifle
« Reply #2 on: July 16, 2007, 06:45:39 PM »
DD,
" We have admired your rendition of the Army version of the 42 pounder during the discussion on recoil reducing and maneuvering gear. Referred to as Seacoast Guns, Patterns 1840 and 1845, they were not designed with rifling in mind. George is correct; this came later with experiments in 1856 to around 1859 and semi-production after that point. True production of the "James System" rifling started in 1861 for obvious reasons. The 32 pounder and 42 pounder guns were favored for conversion by the north and the south because they were so numerous and after conversion were called 64 pounders and 84 pounders because of the new rifled bolt weight. Most were not banded, but this was risky, of course, as accidents would prove. These guns were supplied to a variety of commanders in completely different situations. Some went west to arm the City Class gunboats like the Cairo and some went south to provide breeching batteries for the Federal investment of southern-held forts like Fort Pulaski near Savannah, Georgia. There are 4 rifled, unbanded 32 Pounders at Fort Gaines, AL today and they were fairly common in Confederate wartime installations and on naval vessels.
The Cairo guns recovered in the 1960s included at least three rifled 42 pounders, a pattern 1831 with no muzzle ring and a breeching ring above the cascable, 9 lands/grooves, (RH) and two rifled pattern 1845 which look exactly like yours. They also had 9 lands/grooves, (RH) and all three were unbanded. We have never run into a pattern 1841, nor have we been able to find that designation in "The Big Guns" by Olmstead, Stark and Tucker. The pattern 1840 is almost identical to the 1845, with the exception that it was unturned whereas the pattern 1845 was completely lathe turned. We agree with George, your best bet for a photo would lie with the Nat. Military Park people in Vicksburg, MS. who maintain and interpret the Cairo site."
Hope this helps,
Tracy and Mike