843 is just a code to indicate that the bezel and case are a matching pair. You will sometimes see this with cases and case-backs as well. I have a number of Longines watches with matching codes in the case-back and on the back of a lug.
I have heard that this is particularly common for military watches (although the OP watch is not military), because military watchmakers were often working on many identical watches at the same time, and it was thought that keeping parts paired up ensured that they would fit correctly. Ironically, many of the military watchmakers ignored the numbers, and just assembled watches haphazardly. It's one of the quirky aspects of military watch collecting.