But regarding the Hangman acetate John, you mentioned in your original post that "these acetates were traded on the open market 20 years ago" (possibly not verbatim).
Now that to me implies that at least a handful of these (it obviously could mean just two, but my ears hear "quite a few") were circulating back then. Surely only serious collectors would have picked these up? They're not gonna lose these or damage them in any way, are they?
These copies have to still be around, treasured by serious collectors like yourself, and then the problem is simply one of QP locating these people.
Or is bullock-faeces emanating from my oral orifice?
Gullibility and credulity are considered undesirable qualities in every department of human life -- except religion.