It looks the same as the G&G AEG lowers, what's your problem with getting it back in?
The front pin on the AEG's has a slot in it that a small pointy pin tensioned with a spring pushes into to stop it coming all the way out of the receiver. If you get a jewellers screwdriver, you can push that pin in by sticking the screwdriver in the slot and then pull the main pin all the way out.
To put it back in, just stick the pointy pin back on the spring and push it in with the jewellers screwdriver and then slide it out of the way as you push the main pin back in.
I just did this yesterday, it wasn't too much trouble.