1. No idea
2. .1 mm is roughly. 004 of an inch, so .2 difference is. 008 constriction and.5 mm is approximately 20 thou. In 20 gauge I would call that improved cylinder and improved modified, or "perfect".
3. The early sixties are when AyA established their reputation with King Bros. importation of their guns into Britain.
4. Are we talking the same gun here? Rounded action Number 2 guns are not from the 1960s in my limited experience. To quible the AyA is a rounded action, not a round action anyway.
My 1966 AyA hand detatchable sidelock 20 gauge is not rounded, has similar chokes, 26 inch barrels, very minor hunting marks on wood, 100% case colours, and I value it at $3500, which is what I paid.
Since AyA guns are as varied as the purchasers specs I would not rule out any variation with certainty.
