this is the Quick Release Base, similar to the otto nemenz version of the ronford baker quick release bases (RBQ).
The QRB is compatible with all large size quick release systems like the O'Connor 120mm euro plate and sachtler 35 camera plate.
I also sell the small size (mini-QRB) as well as touch & go plates for both sizes.
Dimensions of the the Quick Release Base:
4-1/2" x 6-3/4" x 11/16" weight 1lb8oz