One reader of my blog send me this scan.
The overprints are looking good. They are most likely genuine – the framed Z looking like E6. What makes those items rare is the combination of framed and unframed Zs. This is most unusual and my guess is that someone made deliberately. This is not a random mistake when producing the overprints. The ink color was changed too. I wish I could take the measurements of the framed Z to be sure it is genuine. Perhaps this is Serebrakian work. If there is a SER signature on the backside (or part of the large lilac one) – we would know for sure.