It is designed to fit single and double glazing. It has the added feature of the flap being able to swing higher to reduce the problem of broken flaps. 

Generous flap size: 300mm x 270mm (12″ x 10 13/16″)

Overall size: 430mm x 415mm (17 3/16″ x  16  5/8″ )

Application: Suitable for single and double glazing. Glass thickness 3mm to 32mm (1/8″ – 1 1/4″)