Originally thin sheets of
Mica was used as electric insulation for power semiconductors, occasionally it still is actually. If the Mica is frayed at the edge, it might at a glance perhaps look a bit like a piece of cardstock, damp with some liquid or other.
If this is a PSU you intend to use a lot, then I would suggest perhaps rethinking the cooling of the 78xx/79xx devices. The back of my envelope claims each would dissipate around 10W or so of power when delivering 9V @ 1A, and the back of the chassis will almost certainly not be up for the task. It probably wont outright kill the regulators if you just want the power for brief moments at a time, as they will just shut down when hitting their temperature limits. But it might be annoying if you intend to power more substantial things for long periods of time.