This is information on the three cast iron Army 42pdr conversion rifles that went down with, and were later recovered from the U.S. Gunboat Cairo, as taken from page 165 of the National Park Service document (that DD already posted), and the National Registry of Known Surviving Civil War Artillery.
Location on the Cairo: Port bow (when looking forward toward the bow, the left gun of the three bow guns) - Marked on right trunnion, K & W (Knap and Wade, foundry owners names), F.P.F. (Fort Pitt Foundry) - Right rimbase, 312 (foundry no.) - Left trunnion, 1856 (year tube was cast) - Breech, 8,359 lbs (weight of tube) - Muzzle face, No.8 (U.S. registry number), B.H. (barrel inspected by Benjamin Huger) - Top of barrel between the trunnions marked U.S. - Rifling, 9 grooves right hand twist. 42-pounder seacoast gun, Model of 1845; smoothbore converted to James system rifling.
Starboard bow (right gun of the three bow guns) - Right trunnion, K & W, F.P.F. - Right rimbase, 324 - Left trunnion, 1856 - Breech, 8,397 lbs - Muzzle, No.20, B.H. - Between trunnions, U.S. - 9 grooves right hand twist. M1845 42pdr converted rifle. From "The Encyclopedia Of Civil War Artillery":42-pounder seacoast gun, Models of 1840 and 1845. Total length, 129 inches; weight, 8500 pounds. While orders, inspections, and deliveries are recorded as two separate model years with separate series of Registry Numbers, there is no dimensional difference between the two models. The only disparity is that Model of 1840 are lathe-turned only at critical components (muzzle, breech, trunnions and rimbases) while Model of 1845 are fully lathe-turned. Total production: 40 Model of 1840 by Columbia and West Point foundries from 1841 to 1845; 318 Model of 1845 by Alger, Bellona, Fort Pitt, Tredegar and West Point foundries; known survivors, 3 Model of 1840, 29 Model of 1845. More than half of the known survivors were rifled by both sides (some also banded) during the Civil War.
1st starboard broadside gun - Right trunnion, J.M. (General John Mason, owner of foundry), C.F. (Columbia Foundry) - Left trunnion, 1837 (year tube was cast) - Breech, 8,620 lbs (weight of tube) - Muzzle face, No. 26 (U.S. registry number) - Top of barrel between trunnions marked U.S. - Barrel inspected by James Wolfe Ripley - Rifling, 9 grooves right hand twist. Model 1831, 42-pounder SB converted to James system rifling. From "The Encyclopedia Of Civil War Artillery": 42-pounder seacoast gun, Model of 1831. Total length, 129.4 inches; weight, 8687 pounds; total production, 167 by Bellona, Columbia and West Point foundries from 1831 to 1840; known survivors, 8. Its single 52.2-inch reinforce and breeching ring are distinctive features.