You have certainly researched this thoroughly. I can't think of anything else you could have done. I think this is now a situation calling for transcriber judgment. I think some transcribers would simply omit this unkown symbol that has no print reference and I would support such a decision. On the other hand, I know there are also transcribers who would write a TN. Since you have gone to the trouble of determining that this unknown symbol appears only once, I would consider a [u]brief [/u]TN preceding that entry that says the unknown symbol after the following entry word is omitted in braille. Your choice!