I’ve used this photo a few times now. It is a 30-pdr Parrott rifle at Lee’s Hill at Fredericksburg.
The carriage is a metal reproduction, replacing a wooden carriage which had badly deteriorated (and left the 4200 pound gun in an unsafe condition). The carriage is a close copy of a siege carriage. You see the rest or cradle on the stock and the traveling trunnions on the cheeks, which were used to situate the gun to the rear of the carriage while on the move. Notice also the carriage lacks the loop at the base of the trail, seen on the smaller field gun carriages. But there’s another fine point of detail to note about this carriage mounting.
This Parrott carriage lacks the standard elevating screw and four handled head that is seen on most reproductions. Instead there is a long threaded screw, offset to the left, and a single crank.
Notice how the screw is fixed with a single bolt to an eye on the carriage trail (bottom of this view). The single arm and handle at the top of the screw provides clearance for the gunner. The screw rotates through a brass nut. That nut has a pin fitting directly into a piercing within the knob of the gun.
Very simple in concept. Turn the elevating screw and the travel of the nut will impart elevation on the gun. The Navy and Army used elevating screws of this type on heavy guns. A good example of such wartime use is this photo from the Washington defenses.
The Parrot rifle on the left has the single handle elevating gear offset to the left.
Just a fine detail point to consider on your next visit to Fredericksburg. My contribution to the “how did it work” file for the day.