I've only heard of these and not seen one. From what I've read, these are just about the most rare stamps, at least in terms of US stamps, and most people consider owning them necessary to be considered a "complete" collection of US stamps. Anyone seen one in real life?
Yes, I've seen the common Z grills while examining some lots at auction. I haven't seen the rare ones like the 1c Franklin, 10c Washington or the 15c Lincoln. But the 2, 3. 5 and 12c varieties are not that rare. I have seen them up close.
