Larson corner piece

This appears to be a screen panel corner key, but I unfortunately can't say much more than that. The only way to identify this kind of corner is to remove the original and take precise measurements of the leg size (width and thickness). It appears to be stamped in the frame, too, so you have some work ahead of you just to get it out. I'll share a video that shows the proper method of removal, but ultimately you may want to contact a professional to do this for you.

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