Andrew, It�s not a date so I would go with an accountability number of some kind. Comparable rivet variations are seen on military issue frogs for combat bayonets. And I have a new condition (unissued) combat bayonet frog with the same teardrop shape for the frog stud hole, dated as I recall 1940 or 1941. The frog looks OK from the image, but I would also see if it smells like old German leather - just to be sure. Regards, FP