250-500cc Dumb question about ordering correct piston

So my dealer told me to bring in my cylinder and piston since I have it off. He was going to measure it for free to ensure I buy the "correct" one. Well, when I look online you get the option of an a b or c sized piston. My cylnder is clearly marked with an "A"

What am I missing? Can't I simply place an order for an "a" piston or is there something (tolerances mabye)special my dealer has to check?

that would be correct if the cylinder had work done to it over the years, but as I recall you recently posted this is your first piston and if there is no cylinder damage then it should be an A piston replacement