The 11687 is the assembly floor number. It was used to keep major components together during the early stages of production and before the true serial number was stamped.
Almost invariably the assembly floor number is 5 digits. Why, I don't know. As Jimbo notes, once the gun leaves the factory, those numbers are meaningless.
If this is a 6-shot .38 Special, which I believe it to be, the serial number, I THINK (off the top of my head) would put it's production date sometime between 1910 and 1920.