This is a component part of the EK3 rail system. You should only order this part if you need to mount an existing EK3 rail system to a new G&G or Army Armament airsoft gun.
This part is the sight rail which creates a continuous 20mm rail along the top of your airsoft gun. It mates with the front rail system to give over 30cm of rail 20cm space along the top of your L85A2 to mount sights, sight protectors, etc to.
The correct mount block for use with this sight rail is order code 100321 - EK3 Mount Block (G&G/Army Armament)