Discussions are ongoing: I have discussed with Joseph Geyfman and he has discussed with Alexander Epstein ... We are still talking about probabilities rather than certainties so we really need some new evidence. See also now comments below from Ivo Steijn.
UPDATE 30 May 2018 Tobias Huylmans offered to look at the item using his specialist BPP equipment. He could not get a definite result. This is partly because the inks of the overprint and the canellation do not show the kinds of fluorescence which would enable him to get a contrast. So we still need new examples of this overprint before we can make any progress.