1840 50c Reverse Of 1839 - What Do You Think (PCGS)? - GTG

In another venue Rich Uhrich mentioned having seen only one other example aside the two PCGS lists, and that fifteen years ago. It has to have been known at least that long. Since PCGS attributes not only by Wiley-Bugart numbers but also FS from the Cherrypicker's Guide - and this coin has both - whichever became listed by one of those sources first is when PCGS was able to attribute it. Of course, you're right, there are almost certainly other examples out there which haven't been attributed but Seated stuff has a pretty loyal following and I can't imagine too many examples being unidentified.