They’re the original Newton Undo/Redo buttons. Sprang wanted to pay homage to the Newton, so he used the icons in his now-famous app.
Here’s the original Newton version:
As you can see from the above still frame, the Newton icons remain in the new iPad version of Brushes (in the keynote, you can see them at the 42:30 mark).
Sprang developed apps for the Newton, too, back in the day – including Lathe, a popular 3D modeler.
It’s been gratifying to see Sprang’s success with his Brushes app, and great to see he still uses those Newton icons from way back when.