The Missile Launcher is a cut Serious Sam 2 vehicle.
The Missile Launcher is a turret with two large green boxes on it. Inside each box is a large amount of rockets. Based on footage, the Missile Launcher would fire a rocket from one box, then very quickly fire a next rocket from the other box. It seems that the Missile Launcher would have a fast rate of fire.
The Missile Launcher managed to survive long enough that it was mentioned in the manual. Why it was cut is unknown.