External Window Screen Frame Corner Key with Nibs and Flange for Square cut 3/8 x 3/4 Frame Material 
Used by Weather Shield Visions 1000, 2000, 3000 series vinyl windows and by many others 

Application: Holds the 4 corners of the window screen frame together
Used with 3/8 x 3/4 aluminum frame material
Corner Outside Thickness: 3/8 inch
Corner Inside Thickness: 5/16 inch
Length: 1-3/4 inch
Material: Nylon/ Plastic 
Color: White or Tan

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.