The 
HK Hostile LTS Air-Link Tank Holster in Black
 is an airsoft accessory designed to securely hold 13ci and 15ci HPA tanks horizontally on compatible harnesses. The product listing is accurate: the holster itself is sold separately, and the harness is not included. 
 
Key Features
Compatibility: Designed for seamless integration with the HK Army SpeedFlex and Synapse harnesses via a secure Velcro attachment system.
Tank Size: Specifically accommodates 13 cubic inch (ci) and 15ci compressed air tanks.
Design: Features a compression elastic tank sleeve, a hardened back plate with anti-slip coating, and a protective adjustable cover to keep the tank safe and stable during movement.
Additional Features: Includes left and right hose guides for tidy line management and two integrated Mag Cell holsters for quick access to rifle magazines.
Note: The harness, tank, and mag clips must be purchased separately.