This is a 1" spacer that can be used to help mount your alternator. It is especially useful if you want to replace an alternator with a 2" mounting foot. These are sometimes found on Westerbeke engines. The spacer makes up the extra difference between the feet of the Balmar alternator. Another use for this spacer is if you need to offset your adjustment arm to line up with the face of the alternator.
This is one of a set of three spacers available, there is a 1/4" model (p/n 10-4048), a 1/2" model (p/n 10-4000) and a 1" model (p/n 10-4047)