The 10.50" top opening is for the newer roots blower top openings, the 12.875" works well for older blowers and the plate has been drilled if the restraint needs to be opened up to 15.00" for older roots blowers with the Buzzard opening.
The double bolt pattern also can be used for running a 13.250" on bolt center injector on a large opening 15.250" on center blower.
Racer can choose on the drop down menu for the opening in the top plate. Shown in the pictures is the 12.875" opening. CURRENTLY OUT OF STOCK ON THE 12.875" RESTRAINT.