1 3/4 inch ( 44.5mm ) to 2 1/8 inch ( 54mm ) outside diameter vacuum adapter

Originally created to adapt a DeWalt shop vacuum to fit the Cen-Tec Systems bayonet but I find it fits adapters around the shop. 

Opening 1:  1 ¾ inches or 44.5 mm outside diameter

Opening 2:  2 1/8 inches or 54 mm outside diameter