The only ones that must be conversions are the civilian ones that are in the NFA database, for the reasons I mentioned. LEO, Class III dealer samples, etc, can all be real MP5s.
Conversions and real MP5s are not made from the same parts. This is becuase in order for the HK-94 or SP-89 to be imported into the US as a Semi-Auto weapon, it had to meet BATF approval to NOT be easily converted to full-Auto. Therefore, the conversion to full auto could not be as simple as parts replacement. It needs custom parts to be made. So to use your own Z-28 example, the mechanic is not really using genuine Chevy Z-28 parts for the conversion.