This is the Motor Mount Plate.  It can be used with both V-Slot and OpenRail.  
The idea behind this plate is to give you a mountable yet movable plate for your Nema 14 & Nema 17 Stepper Motors.  This works great as it allows you to easily mount your motor anywhere on the extrusion.
There are a lot of possible mounting configurations for this plate.
This design was inspired by Barton Dring of buildlog.net
6063-T6 AluminumBlack Anodized Finish3mm~ (1/8") Thick 
Belt Driven linear actuator example and Plate Models
Example Build
Please note: Other part models in varouis formats can be found in the Resources Parts section of OpenBuilds.com