How Many Distinct Shapes When Omitting Two Cubes from a 27-Cube Stack?

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Suppose 27 identical cubes are glued together to form a cubical stack. If one of the small cubes is omitted, four distinct shapes are possible; one in which the omitted cube is at a corner of the stack, one in which it is at the middle of an edge of the stack, one in which it is at the middle of a side of the stack, and one in which it is at the core of the stack. If two of the small cubes are omitted rather than just one, how many distinct shapes are possible?

 

[Bacle]

Distinct up to what, rigid motions, i.e., shapes s,s' are equivalent if s' can be obtained
from s by a sequence of rigid motions? Maybe isometries, etc.?