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How do you tell it's reperfed?
Since there are a couple sites out there that explain, using various methods, how to tell if a stamp has been reperfed (had it's perforations redone or retouched or even added) I will only mention the basics here, being no expert at all myself.
Telling a stamp has the possibility of being reperfed I think really comes with experience and time. A reperfed stamp just strikes you the wrong way.
Basically you are comparing how the perforations (perfs) on one side of a stamp compare to the opposite side, or top to bottom, or sometimes both. Do the perforation tips and valleys line up? Has one side a different perf than the other, when you would expect both to be the same?
Also you look for the evenness of how the stamp was separated from it's neighbour stamp. Too even is a warning sign, not necessarily the final deciding factor, but something to keep an eye out for.
Knowing what can be expected in one stamp from another, the varying perfs used by the printers, what different papers do when improperly treated, having examples of properly perfed stamps to compare possible reperfed stamps to, even having pictures, these all help.
These two sites can help explain reperfed stamps also:
http://www.hgitner.com/whats-new/70and one referenced by Ray mac on Stamp Community:
https://goscf.com/t/21600