From a prism screen

I'm afraid that your corner key is no longer available. Take a look at our 95-186 to see if the dimensions match up to yours. 

Looking at your original corner key, it appears that yours has a "stop" on it. You will need to install a small screw in the corner to serve the same function, assuming our 95-186 is the same size. Sorry we couldn't get you a better match!