Bikemonger says.... As the name suggests, an elongated cargo cage which is great for carrying longer things. Tent poles and wine bottles spring to mind. 

It's got four stamped titanium back plates spaced so you can use 3-pack mounts or standard bottle bosses. As you're unlikely to have enough bolts on you frame to use all four, each plate is drilled to accept accept a hose clamp with an 8mm band (available for pennies from hardware shops) giving a great variety of mounting options.

Works perfectly with Voile straps (not included). 

Dimensions (in inches because they're American): 2 1/2” wide x 15” tall, and the shelf sticks out ~ 2 1/4”
Weight 80g

Made in the USA