Spanish Stamp Expertiser Mark?

I was going through some duplicates and I happened to notice this mark on the back of one of my stamps. Anybody know what it means?



This is the stamp it came from.

I don't find this on the list of "Expert Marks." It is possibly an owners mark. Scan the back of the stamp It's location can shed additional light on the imprint.

Hey Rev. The mark is on the other side of the big 1.

It has to be an owners mark.
Or it would be telling you it was a used stamp.

If you are using the location convention for expertizers' marks, wouldn't the mark also be telling you it was imperforate?

(Assuming that the design is intended to be an "S" in the orientation shown in the scan?)

It doesn't seem obvious enough (this far removed from when and where it might have been sold) to be a dealer's mark, so I would suspect an owner's mark, too. Is there something more interesting about that stamp than is immediately apparent? I just have the basic Edifil, so I don't know of any flaws or varieties.