Don't know if it has any connection or not,,but the use of Roman Numerals was used by the Spanish gun trade going back at least into the late 1800's to match up parts on a particular gun. Sort of an assembly number to keep all the major fitted parts of the gun readily identifiable as the parts went through the different mfg and finishing processes.
Star and I believe Llama still used it up to the 1960's and later in some cases.

Perhaps a clue as to who W&K grip makers/carvers learned the trade from.