Gail,
I think you need to sort out some basic facts to make any progress. Where did your father change his name? In
England it is legal to use any name as long as there is no intent to defraud. However if something like a passport is required it is likely that the original name would have to be stated as there would be no bc under the assumed name - so if your father changed his name in
England and then applied for a British passport you might find a clue there? Why would he change his name to "protect his family"? Stein is not an uncommon name in Germany, nearly 17,000 entries in dastelefonbuch, so he could have belonged almost anywhere. Was he a Jewish refugee? Stein is not unknown amongst Jewish names. As to Prussian princesses I would be inlined to doubt that. Most of the Prussian royalty stayed in Germany, they were not turned out and in any case Germans were most unpopular in
England after WW1 - even the present
Royal Family had to change their name because Germans were so disliked. Or did your father go straight to Australia from Germany where things could be quite different? In that case you would have to look for Australian immigration records. I would suggest your first step is to let people know where and when he changed his name then someone might have a suggestion for the next step. Just for interest,
Prussia did not "fall". Germany became a republic at the end of WW1 with
Prussia remaining part of the state. The name "
Prussia" was officially wiped out by the Allied Control Commission in 1947 - not that I think anyone was particularly worried about it by then!