It's the case number that's matched to the caseback. It was usually done with snap backs but the practice may have been kept on.
Silly me. I never even thought to compare it to the caseback. Was it a common practice to have the caseback and cases match exactly? For military purposes? I mean a caseback from a 145.022 Speedmaster for example could be swapped on to many others of that same reference.
Fantastic dial!