I think the arch is exaggerated because the lack of internal structure makes it unnecessary to have a a lot of thick wood there. Weight-saving measure.
also saw the scale on the bottom pic, looks eight feet to me, kinda big for a swivel gun.
You're right, it's a bit large for a swivel-sized gun (hence my edit).
I imported the drawing into my CAD program, enlarged it, set the program to match the scale as near as I could, and pulled a few approximate measurements:
Height from the bottom of the sled to the center of the trunnion = 28½"
Length of the cheek mounted on the sled = 40"
Distance from the center of the trunnion to the center of the elevator screw = 27½"
Distance between the inside of the trunnion caps, at the center of the trunnions = 7¾ - 8"
Trunnion diameter = 2¾ - 3"
(scaling up makes the lines too thick for accurate small measurements)
So whaddaya think? 3-pounder?