[fuzebox40]

That may be the manual hold open. The charging handle has a little knob on the opposite end that fits into a little hole on the left side wall of the receiver and thats how it's used (just a little push). It's pulled back most of the way at that point but the "last shot hold open" only holds the bolt about half way back.
The mechanism separates from the receiver as a unit so it's all super easy until you have to disect the mechanism itself to replace a spring or something. Some people recommend taking a picture of it first for later reference if it needs total break down. Worst thing about it not working is dry firing the gun when it's empty.