The only versions I know of were the original one, which has no thumb safety, and the later one that has. The safety was reportedly put on at the request of the Luftwaffe, which subsequently purchased many of those pistols. (Note that the safety is "backward" from that on the Walther and Mauser pistols, moving downward to the fire position.)
It is a good gun, and even the very late ones were pretty well made, though rough on the outside. The eagle is the German army acceptance stamp. The chrome plating and ivory (probably plastic) grips were put on the gun after it left German service; no German service guns were plated with nickel, chrome, silver or any other metal, or had other than issue grips.
The caliber 7.65mm is what we call the .32 ACP; ammunition should be available at any gunshop or sporting good store.