You're welcome.
The easily made Roman Numerals were undoubtedly one reason the markings system was chosen IMHO. Done quickly and at times crudely it was still a quick and positive identification method.
I've never seen one in steel that was done with purpose made letter punches/stamps. Mostly scatched into the surface(s) with a pointed scribe or in some cases the letters made quickly with just a straight chisel edge used to form out the letters with light hammer taps.

Also R/Numerals were universally recognized and even the less than educated could most often still read them or at least match them up to each other and their commonly used steel parts assembly trays that were also marked.
The numbers range at times was quite high as the parts lots went through the mfg processes with makers like Star, Llama, etc. But as the guns were finished and out of production, the numbers/trays were recycled through the system again.

Also, some small makers & one-of makers (alot of the industry was still cottage industry) may have chosen an extremely high numeral prefix for a personal identification of sorts. They were outsource workers in many instances and their name didn't appear on the final product. I wouldn't doubt if some of those grip makers had done the same to leave a personalized mark of sorts.