Cool, I don't know much about 9x21 except that it is longer than 9x19 
The cases are, but the cartridge overall length isn't.
I believe the number on the right side of the frame is the original serial number. I believe the number you covered up on the left side of the slide is the importer's serial number, since, being 9x21, the gun was probably originally sold in Europe and later imported into the U.S.[/quote]
The numbers on the barrel, slide and frame that I covered up are definitely the factory serial number. I have multiple other CZ75's with serial numbers in the same location. They as well as the 5874 were all engraved prior to the pistol being finished. You can see where the importer engraved on the front left of the slide. They kind of buggered up the finish when doing so.