Basic idea by Stewart Coffin, made by J. Storer 1989.
(purple heart with light wood dowels, 3 inches)

Stewart Coffin proposed this class of puzzle to make a 2x2x2 cube from 8 unit cubes, where each has three mutually perpendicular holes, and a total of 12 dowels are inserted into 12 of the 24 holes. He observed that the holes have one of two "reflexive forms", and that puzzles could be made by having all pieces of one form or having 4 with one and 4 with the other. The cubes of this puzzle all have the left form; the photos show basic solution steps: