> This masterpiece by an unknown artist
Not unknown. It is signed "G. Clement", for Gerald Clement. The title is "Lesson 3d" because it actually is the third lesson in a tutorial for MacGrid, a software developed by Clement that teached people how to draw in MacPaint.
Wow, nice find, thanks!
Here is the next picture, "Lesson 3 E. Morning Light":
https://ibb.co/8DkdrG9q