Sounds suspicious. Are you sure you aren't talking about an assembly with boards and a case? Bolts aren't normally
considered vias. ;^)
No, I'm talking about a narrow board with two rigid parts at the ends and a flexible part in the middle. The rigid parts had
more layers: that's how they got to be rigid.
It lived inside a cryostat, with circuitry that ran cold at one end, and circuitry that ran warm at the other.
The point here is that the insulating planes are layers also: they need representation in the board description. They have
their own shapes and material properties. And I think the only way to do blind/buried vias, buried components, and odd boards
like this in a non-kludgy way is to treat the insulating planes as layers, each with its own geometry.