Actually I disagree. Those edges are too irregular to have simply been torn. The shade and impression are right for an imperf, and the paper is reasonably thin. Imperfs were being used in California much later then this, so it is certainly possible that is where it was used. I do agree that this is not the typical "sewing machine" type of perfs, but someone could have used some kind of course device to "perforate" a sheet or large multiple. I can't see any way that this stamp could have had regular perfs and wound up looking like this.
Southpaw the irregularity could have come from the use of a tracing wheel. I do not think it was put against a steel rule or edge and torn as it is much rougher than the other imperfs I have seen and have been told that were the means used. 1866 is also a bit of a late date an imperf. its just another mystery in the vast revenue ocean.