It looks like you have the same mount as I have on mine. When you take the gun off and replace the armour plate, you can flip up the tail and the mount rests on the bent tip of the plate and the wheels. In this position you attach the gun to the end of the mount for anti-aircraft firing. Did you take the cleaning brush/caps off your grips or are they missing?
By the way, mine is CA. It's sad I can't shoot it anymore, but it was a real workout as you have to cycle the action manually between shots.
Here is a crappy shot of mine sitting on the floor of my gunroom (bottom left)
Another shot having fun with it in my back yard. My father is the driver. This shows it in the AA position (ready for deer hunting, redneck style)