I found no cracks or loose fittings.
DD, does your photo show a nut missing from one of the cheek's rear tackle eyebolts?
According to the information that Elizabeth Joyner emailed you, the carriage she lists was the mount for one of the M 1845 42 pdr rifled conversions that were originally cast in 1856, and she mentions a cap-square key chain, so you know that at least one of the carriages had the keys attached by a ring to a chain that was itself then attached to a smaller eyebolt ring on the side of the carriage cheek. Did W. Green's drawings show this feature? Considering the fact that you have never lost one of your keys after all this passed time, and the amount of shots fired, is kind of amazing; you must have really had those keys wedged in the cap-square bolts tightly.
Catalog # VICKC 2134 (Carriage)
Wood (white oak) with wood and iron fixtures (painted black); screw elevation system.
LxWxH (excluding wheels): 1.867 m x978 mm x .686 mm. Radius of wheels: 102 mm. Slightly checked. Right front wheel missing.
Right key missing from chain. Screw handle missing. Salvaged with USS Cairo. Was originally located in the U.S.S. Cairo port bow position.
Catalog # VICKC 2119 (Tube) 42-PDR
Iron. Rifle. Marks: right trunnion: K & W/F.P.F. Right rim base: 312. Left trunnion: 1856. Base of breech: 8359. Muzzle face: No.28/B.H. Top, between trunnions: U.S. Pitted. History: salvaged with U.S.S. Cairo; was port bow gun.