This is the Electrode Holder from the Octabolt assembly. It allows Ag/AgCl coated electrode combs to be mounted with a screw, and aligned with the spring inside the Octabolt. These are included with every Octabolt in the "Print it yourself"
Ultracortex kit in the OpenBCI store, but a longer one is also available here (Choose "Large" size). The screw must be replaced with a longer one, but along with the longer Octabolt, this allows for up to 25mm extra extension.
Update August 8th, 2017: Samples showed bonding inside Eholder screw hole using Nylon SLS (over sintering). Design has been updated with extra clearance. Please contact me if you need replacement parts.