You have been here correct?
https://en.wikipedia.org/wiki/MouseText
I can't prove it but I suspect that the Bánffy, Ricardo cited in reference 3 and 4 is the rbanffy seen here and on the orange site. maybe these are bread crumbs to help your quest. apologies if you knew these details already.
For example, the screenshot in the 1985 Apple II HIG shows something like the character at 5E (in the table on Wikipedia) being used for the close box at the upper-left... but is that intended? The character at 5E doesn't have a left-hand edge, and I'm guessing that it's supposed to be used next to the character at 5A to provide that left-hand edge, but that's just my guess. I'd like to find some more reliable source.