New old stock, original Minarelli 38.6mm piston kit.
Most of your V1 engines have a 38.7mm or 38.8mm piston. This one is a little smaller than the rest so that's unusual. Otherwise, the dimensions are the same.
From the edge of the crown to the center of the wrist pin opening is 29mm.
Total length from the edge of the crown to the skirt is 49.5mm