Note that 65mm converts to 2.559" while 70mm is 2.756".
2 9/16 " = 2.5625" so 65mm is only .0035" shy while it is .059" longer than 2". I personally call a 65mmm chamber a 2 9/16" one rather than 2". I also believe that most British guns, except very early ones, had the chambers cut from 65-67mm (2 9/16"-2 5/8"). This is covered in W W Greeners book "The Gun & its Development".
I have the 9th edition (1910) edition but much of it is carry-over from previous editions so I do not know the exact point in time this took place.
My pre-1900 J P Clabrough has 2 5/8" chambers, but it was made primarily for the American market. I have never checked the chamber length on my ca 1863/65 W&C Scott pinfire. I need to do that.
