This video from RogueSystem087 was posted last December and shows the various railed gas tubes for the GHK AK Gas Blowback Rifles. Made by Hephaestus Airsoft, the company usually cooperates with GHK for upgrades or unique airsoft projects. There are three versions made for the AMD65, AKS74U and AKM/AK105/AKS74 GBBs.

Hephaestus Airsoft announce the release of their AKS-74U/AKMSU Railed Gas Tube for AEGs/GBBs... "CNC machined aluminum construction, it is hard-coat anodized in black colour. The low-profile design allows co-witness with most micro dot sights. It is lightweight and is designed as a drop-in replacement for Gas Tube on AKS-74U and AKMSU series."