Define 'modification'? When I think modification, I think of removing or adding metal. That said, you don't need to modify any UZI receiver to use a DES lower if you remove the bolt safety from the lower. There is no 'modification' to do other than removing that part. That is the case with the IMI UZI's as well. I personally have never seen any UZI's in the US that came in with the bolt safety. I know many have modified their UZI's to take that safety in the US but I don't know of any Pre86 guns that came that way. I think that was a newer design change that the Germans adopted.So, for a factory IMI SMG, will the DES lower work without any modification? Also how about converted Action Arms IMI SA conversions? Is it only the Vector / Group RR SMGs that need the modification?