So, apparently on some race engines and otherwise weird engines from the 60s-70s, ordinary head gaskets of the day were just too inferior. So, they use not only a copper head gasket, but an extra metal ring to focus pressure on the copper...

Why did I learn this? Because my dad had to do a head gasket on a Midget with the 1500 Spitfire engine, and some of them have these. Stock, in a *shelf* that looks like a liner edge, rather than a groove. Some bozo in the past removed them, slapped it together with sealant, and when my dad got the head back on (not having known what he was looking at), it leaked like a mofo.

This forum post provided further details on specs, once I knew what I was looking for. Apparently a lot of these blocks get milled flat, because deleting .030 for raised compression and getting rid of asinine wire lunacy sounds pretty awesome.