The existing steel model (prisms mounted on tetrahedron) with cubes of the same volume as the prisms shown next to it, the number of cubic inches of the three smaller cubes adding up to the 216 cubic inches of the large cube below. The exercise that follows on the next page gives a very similar model, as it is closely related; just more compact, and lets say, better proportioned .

Areas: 1 : 1: 1 : 1 :1 : 2 : 2
What Theorem?

The green Yangma. Found this in an old maths book, where it was called a
Yangma. One of the three identical parts of the cube is shown slightly raised. The shapes also appear in Exercise 5.3 in S K Stein's Calculus and Analytic Geometry, 4th Edition, McGraw-Hill.