Dragorn of Kismet gave me a handful of identical mystery chips that might date back to the 1980s. They’re surprisingly large and covered with contacts:
There are no logos or identification anywhere on the chip. The back side is blank silicon.
The visible patterns don’t suggest anything obvious:
The metallization layers aren’t particularly intricate:
Surely there’s something tucked under the top metallization; I have neither the materials nor inclination to dissolve the thing one layer at a time.
I gave a sampling to our Larval Engineer, who says she may turn them into fancy jewelry. I’m sure the solder bumps contain lead, but …