First off, a lot of the 2011 and beyond era HK416s and all the MR556 (US civi version) come with a plastic dustcover.
Second off, there are only two major faults in regard to authenticity with the Iron Airsoft kit: (1.) The HK AF mark on the upper receiver should be white laser etch, not deeply engraved, (2.) like you mentioned, serial number does not begin as 88. It is annoying that the trades are not filled in from the factory, but it's fairly easy to do yourself and get the colors exactly right.