At one time Ruger sent out 50 KSP-931 to police departments in the USA that might have been interested in a 9mm back up. Once the review period was completed the department could purchase the guy at a great savings or return it to Ruger. The ones returned to Ruger got cleaned up, marked with the * and sold at a discount to stocking dealers.
FYI
The D is for duplicate as per BATF. You should only see these guns as old models. Sadly most dealers have no clue and forget to add the D on the 4473. If the box label is marked it should just be a plain D in ink.
The U is for used. Not every factory used gun is marked with the U. I purchased one of the SP-101 that came back from French police and the box was marked but not the gun.