These are spacers that determines the distance of the wheel pulley from the hub. On our first model, the Prototipo (DSS60/DSS50+), a coloured sticker inside the pulley determines the length of these spacers.
Since the threaded M8 holes in the MBS Matrix 2 trucks hanger we use to secure these motor mounts are never precisely at the same distance from the extremity of the axle, we must compensate this difference by having different sized spacers between the pulley and hub to insure the pulley never rubs on the motor mount. MBS' quality control is now tighter and for any board coming out of our shop since november 2018, the 9/16" fits.
Here are the different sizes according to the sticker colour;
Price is for 15 spacers (enough for 3 rear wheels).