Interesting. I have one of these, too. I wonder what then, differentiates a "factory error" unnumbered scabbard from a "unnumbered armorer replacement scabbard" ? The only thing which might do so is if the replacement scabbard was numbered to match the bayonet. No doubt this was certainly the desired result by German directives outlining how the S84/98 was to be refurbished. However, based on the conditions and expediency (like field armorer as opposed to rear echelon repair facility performing the work), the procedure was probably not always adhered to. Of course it is definitely possible the scabbard missed being stamped to match the bayonet, or the matching scabbard either failed a final inspection or was damaged to the degree in-house, it was replaced with a unnumbered example & shipped out. There may be no way to know exactly what occurred.
Here is another poser. I have a 97% original rust-blue S174 | 36. The wood grips & flashguard match each other. The catch assembly matches the blade's internal tang assembly number. However, the bayonet has no external serial number. Scabbard is EuF Horster | 1939 WaA/unnumbered. IMHO this was a field rebuild from armorer spares and salvaged parts, using an entirely new armorer blade with catch assembly as the base, yet no external numbers are present.