Set of four stainless steel axle spacer washers. These are washers that are 1mm thick and it either side of the large drive pulley that sits on the axle. As standard there is one either side but this is provided as a set of 4 so you can space out the pulley by an additional 1mm or 2mm to align the belt properly with the gearbox. 

Fitment of these washers can be seen in the Sinclair C5 restoration video from The Retro Workshop.