The top row shows plates with a narrow locating slot showing that they are meant for a fixed timing magneto. The others have a wider slot allowing the cam to be rotated a little to provide a manually operated advance and retard. A second slot on the cam plate is used to hold the manual advance and retard mechanism. Only one slot is required for this but there are usually two slots to cater for the advance/retard control to be fitted on either side. Even so, some of the plates have had extra slots ground in to allow fitting to other magnetos. In addition, the cam plate on the left has had it's timing slot widened at some point - probably in an attempt to adjust the internal timing by allowing the cam plate to rotate a little further than originally intended. Unfortunately, this type of modification usually results in the advance/retard control pulling out of it's cam plate slot.