Great to find this website and to see that I'm not the only one trying to find out more about Jacobus David Samuel Hirsch. I am a direct 5th generation descendant from his second marriage (1804) to Phennena Catharina Schleper (* July 2nd, 1772 + August 20th, 1838) and still carry the family name since his conversion in 1787. The origin of his new surname lies in the baptism date itself: Since Jacobus David Samuel Hirsch was baptised as a Catholic in Meppen on August 28, 1787, he took the name of the day's Saint, i.e. St. Augustine of Hippo. His godfather was the local judge Carl Franz Morrien. He might have taken his new first name from him.
I live near Düsseldorf in Germany, not far from the Emsland, whose today's administrative centre is Meppen. After a few years, Carolus Josephus Augustin (aka Jacobus David Samuel Hirsch) bought a house in the immediate vicinity of the historic town hall and set up a shop there.
The photo in the first entry (Bolts99) is taken from the book "Land und Leute im Wandel der Zeit an Ems und Hase" by Hermann Friese. There is more information about his origins in the book, but not too much about his parents, yet apparently his father was involved in the skins, furs and horse trade in Lissa.
CJ Augustin himself is said to have been given the nickname “Augustin Hollandtü” in Meppen, this is because he also sold goods in Holland.
In Emsland there are two streets named after descendants of Carolus Josephus Augustin today: Nicolaus-Augustin-Straße in Meppen and Dr.-Augustin-Straße in Dörpen.
I also found a newspaper article about the origins of the Augustin family from 1989.