Was wondering the same thing the other night while watching the movie. Looks like a variation of the LMT SOPMOD. TAPCO makes the M4 stock adapter for an HK91 which can be adapted to an MP5... then add any M4-style stock you want.
The "growth" lol, looks like they added a cut-down SL8-1 long rail to the top (see how it intercepts with the where the front sight blade would be and the 2nd front sight which look very much like a G36C fron sight. Also notice the picatinny slots that are cut where the optic is mounted--typical of the orginal SL8-1 long rail) and then mounted optics on it. I'm guessing it's just to make the gun look more futuristic.
The "growth" lol, looks like they added a cut-down SL8-1 long rail to the top (see how it intercepts with the where the front sight blade would be and the 2nd front sight which look very much like a G36C fron sight. Also notice the picatinny slots that are cut where the optic is mounted--typical of the orginal SL8-1 long rail) and then mounted optics on it. I'm guessing it's just to make the gun look more futuristic.