Note that 65mm converts to 2.559" only .0035" off the 2 9/16" (2.5625") length shell. 65mm seems to be the "Old" standard length of European shells wile it was also the "Old" 16ga standard in the US.
W W Greener stated that at least by the time of his 9th edition book that most British guns labeled as 2˝" actually had chambers of 2 9/16"-2 5/8" instead of the nominally listed 2˝". Some very early guns may well have the actual 2˝" chambers as well as smaller gauges. He also seems to indicate that 2 3/4" was a standard for Pigeon Guns.
Very early on US chambers were basically standarized @ 2 3/4" or 2 5/8" in 12ga, 2 9/16" in 16 & 2˝" in 20.
My J P Sauer & Son/VL&D Knock-About sidelock from ca 1902 has the 70 stamped on the extractor face as mentioned by Raimey. It is of about 7˝ lbs weight so most likely built as a waterfowl gun.
