Here are some notes taken in the Smithsonian's History and Technology Building on November 22, 1986, from the Ames 3-pounder that they have since lost:
Marks: Breechface, lower: 278 (weight)
Left trunnion: 1853
Right trunnion: Ames Co., Founders, Chicopee, Mass
Bore Dia: 3 in.
Trunnion dia: 2 3/4 in.
Mark on upper muzzle face: 116 (hard to read)
Mark on lower muzzle face: Two letters, illegible
No marks on top of tube
Vent was bouched with wrought copper as in a US military piece
Other measurements:
Trunnion rear to basering rear: 15 "
Length bet. rimbases: 6 15/16"
Trunnion Length: 2 1/4 "
Tube length, muzzle face to basering rear: 38 1/8 "
Basering circumference: 24 in. (7.64 in. dia.)
Breech ahead of basering: 21.5" circumf.
Tube length overall incl. knob: 42 1/4"
Like I said, this is one of the smaller ones, I found a significantly larger, heavier one in a private collection.