The grandmother’s hypothesis generated some interest, so here’s an updated version with at least a mildly slicker overall appearance. Feel free to use this, please link to this blog and nudge me if you’re writing something about this topic.
I’ve stumbled over a fantastic reconstruction here by Leif Beckmann under Günter Knittel’s direction. These look much more plausible to me, so I’m taking this in that direction. The site is only in German for now… we’ll see what we ca do about that.