65mm = 2.559"; 2 9/16" = 2.5625" or only .0035" difference, essentially the same. The British always used the nominal term of 2" for their shells, but at least by the time OF Greener's 1910 edition he stated actual chambers were either 2 9/16" or 2 5/8". This was for 12ga, he didn't state whether the other ga's followed the same practise or not.
I have a twist bbl H grade 16ga Lefever with 3" chambers. As to when they were cut to 3" I simply cannot prove, but my money says they left the factory with them. I can think of no later time they would likely have been lengthened to this depth.
