D&A (DiVincenzo & Arienti) was one of several case makers Hamilton used over the years. They produced cases for many of models of Hamilton watches.
Although Hamilton is considered the finest of the US watch makers, they never made their own automatic movements. They began using Swiss movements in the mid 1950's and included automatic movements in their catalogues at that time.
Finding an automatic Hamilton is not difficult. You would just need to verify that the case was made by D&A.