According to Rich Hartzog's list of abbreviations belonging to fraternal organizations and such (which has come in handy for me more than once), there was (or is) a society called the Sons of Pericles that used the S.O.P. abbreviation. However, I Google-Image-Searched them and came up with a very different looking logo (with no porcupine in sight). It is possible that your fascinating and mysterious porcupine has no connection to them at all.
Hartzog's list:
http://www.exonumia.com/art/society2.htm
(Bookmark his site, as it might come in handy with tokens and such, too.)
For now, I suppose "Sons Of The Porcupine" is as good a theory as any.
Very interesting piece. I hope somebody comes up with an answer for you.
