Elsewhere http://www.stampboards.com/viewtopic.php?t=7580 it was described as:
The APS type perforation found a wide use mainly in the combination of photogravure reel-fed presses. A typical feature of the perforation is the left-overs at the back of the stamp!Aktiebolaget Produktion Service (APS): Swedish perforating machine that made pimples, which were then cut away in a lawn mower type action leaving perforated holes.
You can see the left-overs underneath every perforation hole!
to be continued ...