Wow. Thanks Steelcharge. An article I haven't seen and a photo of a Whitworth I haven't seen. That's like finding 2 quarters when I was 7.
Yes, when you've got a gun that has bolts and shells that weigh somewhere between 2 and 3 pounds, and have never heard that Whitworth named them 3pdrs, it's easy to see why there would be confusion. Although there were a few oddball sizes, like 1pdrs, and a 1.92" with no pdr. designation, the most common where the 1.7" (3pdr), 2.17 (6pdr), and of course the 2.75 (12pdr) field guns.