This is a Dolly DV 100 by Sachtler. It is specifically designed for use with Systems DV 8/100, DV 12 and Video 15 tripods. The DV-100 dolly provides smooth and stable dolly action for all 100mm tripods. Features independent wheel locks and is foot controllable that lets you fix the direction of one, two or all three wheels. 

A collapsed length of only 26.0" enhances portability, while the ergonomically designed carry handle is molded into the dolly's reinforced center bracket for optimum carriage strength.