Wow, thanks! Really interesting info.
I still didn't find what I was looking for. I'm seeking a specific piece of info - A relative of mine asked me to help confirm a family legend that their ancestor, a man by the name of Avraham Dovid Popowitz, signed the affidavit to bring the Mattesdorfer Rov to America.
Those kinds of documents are sometimes found here
https://www.uscis.gov/records/genealogy/historical-records-series-available-from-the-genealogy-program but you have to ask them to search for you, and I've heard it can take months to get an answer, which is often No. I think there's a charge, too.
I found the ships manifest when he came over, and in the space where it asks for the name of the person you're going to, it says "CONGREGATION YESHIVATH 1897 PROSPECT PL. BROOKLYN, N.Y." Do you know what was there in 1938? Maybe it was Chaim Berlin, as mentioned in the article below. Was AD Popowitz affiliated with them?
from The American Israelite, Sept 29, 1938
Noted Austrian Rabbi Here In Exile
Rabbi Samuel Ehrenfeld, head of the famous Yeshivas Chasam Sofer in Mattesburg in the Burgenland province of Austria, has arrived in exile. He came to New York at the invitation of the Rabbi Chaim Berlin Yeshiva and Congregation in Brooklyn. Rabbi Ehrenfeld declined to discuss the circumstances under which he left.
The town of Mattesburg, which originally had 110 Jewish families, had but 20 when he left about six weeks ago, he said. It was strictly Orthodox. The yeshiva, founded by Rabbi Ehrenfeld's great-great-grandfather, attracted students from many parts of Europe. The rabbi, 46, is here with his wife and seven children.