Maverickx,
Searching
ebay under China (search "great wall") you can see this stamp design being offered for $5 and under. It is stated to be a revenue stamp and can be seen with many overprints. I did not see the exact overprint you have but you will see several of them for reference.
Googling "china revenue stamps" gives several very nice websites. The following site provides a lot details on this and other sets.(
http://www.manchukuostamps.com/Reve...eatWall.htm)Hobsun