This is the bearing for 16mm wrist pins, which is basically all vintage 180-200cc Vespas.
If you have your stock crankshaft, there should be a number of slash marks on the end of the connecting rod. The slashes on your con rod and the bearing you order should add up to 5. For example, if you have one slash, you would order 151805-IIII
Specs
Outer Diameter: 19 mm
Inner Diameter: 15 mm
Thickness: 20 mm