The importer added ser# as on the K31 is in responce to the BATF importation rules that state:
The imported firearms ser# cannot duplicate the ser# of ANY firearm the importer has previously imported.

Never,,ever,,duplicate a ser# of any firearm that the importer ever imported before in the past,,,ever.

That's pretty high bar when you think about it with the multiple hundreds of thousands of firearms imported over the years, different types, countrys of origin, ect.
Think CAI ever got a gun or two with the same ser# even though they're two different types of firearms

So as not to be caught in that 1 in a million trap (maybe it's less than that!),,the importers decided to, and with the blessings of the BATF, add their own Importer Ser# to many of their wares.

This is a ser# unique to their company and import business, usually unique within their record keeping to quickly ID the type and usually the yr of import of the arm as well as a unique ser#.
It's usually computer applied, dot matrix, quickly and effortlessly and computerized recorded.
No sloppy hand stamping and handling of arms or hand written records when the compliance agents come calling or a Gun Trace arrives looking for a back track.

So that's the why of the Importer Applied Ser#.
Collectors don't like it,,they didn't like the BigBillBoard Import markings,,they didn't like the original tiny Import Markings on the bbl,,they didn't like that the GCA68 stopping Milsurp imports for just short of 20yrs either.