You could have used the World Stamp Finder on Stamp Smarter and found this in 30 seconds.
http://www.stampsmarter.com/WorldID_landing.htmlGo to universal search and type in 'S.O.' and it returns stamps which match this. This tool also supports searching on foreign language characters.
This tool has been donated to APS and they have the source code/database. I am hoping that at some point they will get this running on the APS site.
Don