Wit's End

Old design, this one made by J. A. Storer, circa 1990.
(made from 1/4 inch aluminum plate and 3/4 inch diameter brass rod, 6.5 inches;
the design of this puzzle reflects the figure in Coffin's Puzzle Craft book)

The square fits through the slot but not the ring. The ball fits through the ring, but not the slot. The ring fits through the slot and can pass over the narrow end past the slot, but not over the wide end. The task is to remove the ring. It can seem easier to put the ring on; one can retie the string with only one loop around the middle and first learn how to put the ring on.

3D Printing Of Wit's End
D. Barry has made STL files for a 3D printer:

You can instruct your browser to download these files (courtesy of D. Barry):

Further Reading
STL Wikipedia Page, from: http://en.wikipedia.org/wiki/STL_(file_format)