This is a Dolly DV 75 by Sachtler. It is specifically designed for use with 75 mm bowl tripods. The Dolly 75 provides smooth and stable dolly action for all 75mm 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 25.0" enhances portability, while the ergonomically designed carry handle is molded into the dolly's reinforced center bracket for optimum carriage strength.