[plain OM]

Coolant was leaking from around the radiator cap.

Coolant should not leak out from around the radiator cap. When the pressure in the engine increases, excess coolant should be released to the external reservoir. If it's coming out around the radiator cap, the hose to the reservoir might be blocked or the radiator cap is defective.

If, the overflow hose to the reservoir is blocked, or the rad cap is defective, as the car heats up, the excess coolant is lost (as was evident -- leaking around the cap) and won't be recovered as the car cools, leaving the contents of the radiator down by what was lost.

I'd start by checking the hose to the external reservoir, making sure it's not blocked, that the bottom of the reservoir isn't full of accumulated "gunk", and that the hose fits snugly on the nipple just below the radiator cap. I'd also inspect the rad cap to be sure the rubber gaskets are intact and that the cap is locking down fully on the radiator neck.

 

[plain OM]

The gasket seems to be made of thin steel sheet, and one is rolled around another to make the "ring" around each cylinder. The steel cracked along that "fold", maybe 1/2" long.

Interesting. So is the original head gasket actually two full pieces, with one used to form the rolled around ring around the other at the cylinder opening, or is the gasket just one sheet with a rolled over section at the opening?

Any chance of a close-up photo of that particular area with the crack?