This is one of the common forgeries of the framed Z overprint. While Ceresa distinguishes between several subclasses, for basic checking its enough to know: every thing that looks like this is fake.
The whole Z inside the frame tilts to the left under corner. Easy to spot.
I first came across this first forgery, struck in an intense black ink, sold in 6d. packets of Woolworth’s stamps, made up by Philips if I remember the packet maker correctly, in 1939 along with several other forgeries of Armenian, struck in this same intense black ink. It would be a fare comment to state that any 1920’s Armenian stamps overprinted in an intense slightly shiny black ink are forgeries. The only available black pigment during this period was lamp black, produced by burning a hydrocarbon, such as paraffin wax against a cold metal surface. It was a soft black pigment easily dispersed into a fluid to form an ink of a dark grey-black colour which at high magnification, say 24,000 dpi allows the underlying printed surface, e.g. the basic Arms Type stamp to be semi-visible between the black ink particles. Modern inks, such as those used from about 1935, largely based on aniline dyes, give an intense black ink, shiny or matt (by adding matting agents) which completely cover the underlying stamp surface. So if it is an intense black overprint that completely hides the Armenian stamp to which it is applied – give it a miss as it is almost certainly a forgery.
Ray, thanks for the insight. That is certainly a good hint!
Looking at this scan I can see the basic stamp looking through the black ink, so in this case it is still not too easy…