Third over should be 56.75mm.
Yamaha go in 0.25mm increments, up to 1.00mm OS.
Wiseco do 1.50 and 2.00mm OS, giving you a couple of extra bites.
Be careful with how much you pay for Wiseco pistons from the states, and be doubly careful of buying genuine pistons without rings - the rings are stupidly expensive from your local Yamaha dealer (insert my oft-repeated whinge about IT250 rings that cost nearly as much as a whole Wiseco piston kit here).
Oh, and any YZ125C ~ N piston will work, although you might need to move the ring locating pin in some cases (at worst).
Yamaha gives them all different part numbers because they faffed around with the number of rings, the ring thicknesses, the intake cut-outs, etc - but they're all the same basic piston and they'll all make it go brroooom quite effectively.
