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Until his death in 1999, I used to send John Bulat material for expertising and comment.
At one point, I had some unusual Trident overprints which I found in remainders of the Vyrovj collection - there was a bulk lot at the end of the 1980s Schaetzle sale which I got from another dealer who had bought it and then done nothing with it. Too complicated!
Included were some Poltava type I tridents in an unusual blue - green / dark greenish - blue colour, most of them on the 25 kopeck perforated and all cancelled ZIENKOV (Zinkov): see the scan
Bulat signed these and gave them a - - (rare) valuation in the note he sent when returning them
These blue-green tridents are not, however, examples of the GREEN tridents of Poltava. These really are green. In the upcoming Corinphila sale of the Ron Zelonka collection, there is a cover with a strip of the 1 kopeck imperforate overprinted in green (Bulat 1025) - I have never seen these green stamps on cover before.