Was your gradfather, Herman Katz, born on 3.4.1911 and died 4.6 1946? If so, that he was one of many brothers of my grandmother (my mother's side), Stefanija Kac.
Their father ( and my grand-grand father) was Janez (Ivan)Kac (30.10.1873 - 10.2.1932), married to Antonia Hartman ( 21.5.1877 -27.8.1935).
The fact that you say your grand-grand father was called Yoseph, confuses me a little bit, though this is quite normal with transliteration of names. Do you have the date of his birth or other details? I have details of many members of this branch of Kac , but am particularly interested in the maternal side of my Grand mother Stefanija Kac. They all come from Turiska-Tomaska vas,Smartno, Golavabuka.My grand-grand parents were timber merchants, and had a lot of land. I also have many relatives in that area, and would like to make a deeper research in order to find out where the Kac ( Katz) fammily originally comes from).
Though I speak Slovenian, well, let us continue our corresponance in English. Mayabe some non Slovenian speaking relatives might be able to give us some aditional information.
Slobodanka Teodosijevic - Rome