External Window Screen Frame Corner Key for Square Cut 3/8 inch frame material 
Used by Weather Shield Visions 1000, 2000, 3000 series vinyl windows and others 

Application: Holds the 4 corners of the window screen frame together
Aluminum Frame Material: 3/8 inch thickness
Corner Outside Thickness: 3/8 inch
Corner Inside Thickness: 5/16 inch
Length: 1-3/4 inch
Material: Nylon/ Plastic 
Color: Clay  (Grayish) and Coppertone ( Brownish)

NOTE: 
Screen corners are made to fit certain frame material. Aluminum frame material comes in different thickness and widths. The corner has to be able to slide into the hollow frame, So thickness is important, styles can vary.
Standard Screen Frame Material comes in 1/4, 5/16, 3/8 and 7/16 thickness, and 3/4 or 1 inch widths.