This is the extension wire normally found in an AEG with a stock mounted battery. This extension has a small female connector that attached to the gun then outputs to a large male battery connector.

This wiring extension also includes an in-line fuse box.

You would normally find this type of extension wire on a M4/M15/M16 with a battery located in the stock tube. Also found on MP5 series AEGs as well as many others.