They are pesky little bugs. I'm going to go out on a limb and say that this is either a savings stamps or a revenue. As such, they won't be found in the standard catalogs.
Thank you! I am relieved that I had just not overlooked it in the catalogue!
' Marked "Republic of China Postal Service Bureau" says my source, which might rule out revenues. As might this little treasure that googled-up: https://www.chinastampsociety.org/files/Revenue_Stamps_Dan_Cordwell_OCR.pdf Cheers, /s/ ikeyPikey Addendum: source two says that the characters on the left side read "Savings". Denomination: One Yuan.
