Quoted from MichaelMullen:What is the purpose of this cardboard attached to the heat sink ?
Quoted from AUKraut:Most likely an electrical insulator to stop something metal from touching the heatsink.
It is called fish paper and it is there to insulate the heat sink from contacting any potential non-insulated wire attached to the back of the light (insert) board. When you close the backbox there is limited clearance.
Williams originally used bayonet #44 sockets and stapled them to the wood. The sockets were connected with bare (non-insulated) stranded wire. When Williams switched to using IDC wedge #555 sockets they used insulated wire. The sockets are "insulation displacement connector" so the wire should have insulation to displace.
Quoted from MichaelMullen:There’s a bunch of similar heat sinks without cardboard on the power driver board.
The heat sink on the sound board is a 5299 and 50.8mm high. The heat sinks on the driver board are 5298 and 38.1mm high.