BK2K main and upper playfields.
Many machines need them desperately due to the full mylar sheets that were applied to MANY of the production machines, and make the process of removal not worth the risk, not to mention many machines were beat to crap.
NOS playfields are virtually non-existent, and I have only seen 4 sets in the past 15 years alone.
I don't even know how many overflow runs the factory did, but it was not a lot.