I bought two in total. The differential will not work with this case as the internal gears are jammed inside.
And the central hole on one case is so small that the shaft cannot even fit in it.
All in all, terrible quality!
It's NOT steel!
Whatever kind of light alloy this is, definitely it's not steel. Not magnetic, extremely lightweight only 12g (by comparison the Dremel mandrel is way smaller and weights 23g), and the are traces of yellowish metal dust in the tapered holes. Looks some pot alloy similar to Zama , maybe nickel-chrome plated.
Don't know the strength or durability, will have to test them.