Engine #
The 19S identifies it as a 1.9 litre block - the 75 may show that it was cast in 1975 (some Opel castings do have a year # cast into them.
Unfortunately Opel did not tie the engine number and/or VIN number to the engine number and they are just randomly placedinto different models. So there is no such thing as a "numbers matching" Opel.
However the original engine number was recorded by the factory - it is just real hard to find out what it was other than seeing it for your self.