What is this small triangle on this screw head?
After reading up a bit on screws of various types I noticed this small snippet in the German Wikipedia:

Von 1967 bis 1992 konnte man die Schrauben anhand einer Körnung am Schraubenkopf erkennen. Seit 1992 wird der Schriftzug (SPAX) in den Schraubenkopf gepresst.
(Between 1967 and 1992 the screws could be distinguished by a punch mark on the head. Since 1992 the &quot;SPAX&quot; lettering is punched into the head.) — horrible translation by me.

It indeed seems to be a Spax screw and a bit worn. The cutting part of the thread is barely recognizable as such and the point is not quite pointed anymore. But other features are still there, such as the part of the head that creates its own counter-sink.
So I guess it came from some old wooden parts and I salvaged the screws. Or whoever. No idea where they came from, they were in my random screws drawers in the basement and just happened to be of the right size for what I needed.
When knowing what to search for one can also find images with that marking:

And it seems that some special-purpose screws still have the old marking:

Well, learned something new about screws, that not all wood screws are equal, and that I didn't even have to pre-drill holes for what I've been doing recently.