MP5 upper I believe is what carries the "serial" number and therefore is the part that is prohibited from importation. Whereas with an AR-15 platform the serial is on the lower and therefore lowers are prohibited from importation.
I believe the same goes for the UMP which has a clear upper since that's what carries the serial on it's real steel counterpart.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE