While some early British 12 ga guns proably did have 2 chambers W W Greener stated at least as early as his 9th addition that the normal "Actual" length of a British nominal 2" 12 ga chamber varied from 2 9/16" (65mm) to 2 5/8" (67mmm). I do not have actuals for smaller gauges. 65mm was the basic standard for European guns of the normal gauges. In the early US guns 2 5/8 was a common 12ga size, 2 9/16" for 16ga & 2" for 20ga.
2 3/4" 12ga was also quite common in the US as well. All early 12ga guns with 2 3/4" chambers have not been subjected to a chamber lengthening. I own a 16ga Lefever having 3" chambers, most likely original.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra