In May 2016 a seller on Kobay from whom I have regularly bought Korean revenue stamped documents (especially from the Seoul area) put up a document from Gongju. It is a certified copy of a family register, from 1973. The article was not sold at the time, I have not seen it since.
All the following photos were made by the seller named “jsm654” for the Kobay listing.
Part of the document with the six 20 won stamps:
Other pages from the same document. In the first example the large white spot contains the stamps.
Second example from pages inside this document:
(No copyright infringement intended, these photos are shown as examples only.)