Thanks for your reply. The trunnions are placed approximately mid-barrel, but . . .
. . . I discovered that the bottom of the barrel has additional reinforcement (thickness) in the area shown by the red box in the attached image. This "pad" slopes from the rear of the barrel to the front, where it abruptly stops, forming a well-defined edge (indicated by the yellow line). The highest point on this edge stands about 1/8" proud of the rest of the barrel.
I can't imagine what purpose this served other than reinforcement, but then you would think it would have been all the way around the barrel, not just a small pad on the bottom.
Maybe the sloped pad was part of the elevation mechanism, which I assume would have been little more than a wedge that moved fore and aft on the pad to raise or lower the barrel.
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