Some IPL's are wrong, and some just hard to read, so it's tough sometimes to be 'on the money'.
Cylinder/piston assemblies are different from new.
Sometimes makers will create two different models that are the same basically except,... for some things like better mufflers, cylinder/piston assemblies, colors, carbs, etc.
That let's them have two different price points for more sales. Creating Pro & Semi Pro (Farmer, Rancher,Wood Boss) models.
Good smart marketing as many buyers want something like a Pro model, but don't need the power, or bells & whistles, and price.
However, replacement parts are sometimes reduced to only one type to save inventory costs.
Ie., if a 651SP(Semi Pro) cylinder is needed, the replacement part could be that for a 651P(Pro).
Sometimes both are available for a couple years till inventory levels drop, then only one is made available.
The different pistons #'s can sometimes be based on piston 'fit size', ie., A, B, C,....etc.
Say the part # is, '12345' for an 'A'.
'B' could be #12346, 'C' is then #12347.