The arm is the middle point of the Ultimate Hose Clamp between the Base (Stepper or 90 Degree) and the Attachment.

The arm is not compatible with the spindle base or Onefinity Base (see drop arm). Also available as extension adapters. 

The arm only contains the hardware of the nuts, and the associated screws are included in the Base and Attachment. In these pictures, the Base is at the bottom, and the Attachment is at the top. 

Check out this article for measuring for how long of an arm you need.