Anderson storm window

I'm afraid I can't determine from your photo what size corner you could use to repair your storm sash. I'm also not sure why you don't have a corner key in your frame to begin with. Could it be this storm sash was repaired in the past and the person doing the repair didn't use a corner key? Either way, without seeing the original there's no way to know for sure. Plus, based on how old it is, I don't think that Andersen will still have these corners. Your best bet is to look at our corner key section here and see if you could adapt something here to work based on the size of your frame.