"I believe my grandfather may have been imprisoned in Dachau prior to
leaving Germany for New York in February, 1939 with a visa >from the
American consulate in Stuttgart."
If your grandfather was "officially" imprisoned in Dachau his name
should be on the list of Dachau inmates. A significant part of these
records is already indexed by JewishGen.
Another way to get confirmation is through the Dachau museum staff who
promise to send copies of the records to next of kin. Their email address
I hope that helps
Fritz Neubauer, North Germany