I ordered the original M-Cube about 18 months ago (from Firebox). Love it. Just received the black M-Cube and I am so disappointed. The black balls are slightly smaller than the silver ones. Silver=5. 04mm, black=4. 92mm.
Connect a line of black to a line of silver (side-by-side) and you'll get a curve rather than a straight line. Was hoping to combine the 2 sets to try for larger polyhedra.
Haven't got any rusty ones - haven't got much use for 'em either.
Phil, Banbury - 8th August 2012
Firebox says: Hey Phil, sorry to hear that you are disappointed with the quality of the black M-Cube. Drop us a line on firstname.lastname@example.org with your order number and a few details of the problems you've faced and we will get back to you.