Much like how the SL8 is the neutered version of the G36, the USC is the neutered version of the UMP. H&K had to design it to meet U.S importation requirements. The one pictured has undergone a conversion with an added twist. Rather than using standard UMP polymer magazines, it uses M3 “Grease Gun” magazines.
The benefit with the M3 mags is the 30 round capacity versus the 25 rounds of the UMP mag. They are also considerably cheaper and easier to obtain. Downside though, you lose the last round bolt hold open. This is more for the practical shooter than the collector or purist.