The 2QY and 2QZ are the part sizes, obvious you say, but if i'm correct the Y and Z mean the Diameter of the straight portion of the needle before it then tappers to the point. Y being slightly richer and Z being slightly leaner. do you know what the standard needle size is?

on a closed to quater throttle (and probably all the way to full) this will determine the minute change in fuel distributed into the fuel air mixture and probably lead to a size change in tapper i.e. allowing more or less fuel from close to full open throttle as well.

if there is a number after it, this determines the needle clip position e.g. height. i don't think the 2 signifies the standard needle clip position but i could be wrong.