This is one of the smaller single-cylinder Bosch magnetos and was likely fitted to a motorcycle of some type. It dates from early 1911. The contact breaker assembly and the cam (which can be rotated to advance/retard the spark) is missing from end opposite the drive.
Bosch magnetos usually have a "type" which is described by a two or three letter/number code: ZA1, DU2 etc. Not sure what "side" might refer to.
Cheers
Leon
