The MP5 PIP is another design variant that attempted to improve on the MP5.   It never entered
production either. A straight blowback weapon like the other prototypes, the PIP or Product
Improvement Project appears to be the forefather to the G36 cocking handle, and the Weaver scope
mounting system of the UMP. 
Another fascinating similarity of later design comes in the form of the
MP5 PIP.  Observe the flat cocking handle on top, which appears to
have been adopted on the G36 series rifles and carbines.  This gun
has the sighting system of the MP5, as well as the three lug barrel.