Had this happen on a Lotus engined Cortina I once owned, that also sounded like bottom end knock.
The crack went in an arc down to the piston pin. The piston pin bore was oval, not really noticeable but measurable, so the obvious cause of the knock.
No obvious cause and no overheating. Guesses to cause - lube failure, mistake a on previous assembly or since the fuelling and spark were a bit stoneage it could have been pre ig/detonation. Replacing the piston and new small end bush sorted it, couldn't afford to do a proper rebuild at the time!
I'd wager the same thing here, probably started as a tap (easy mistake for the lifter going) then getting worse.
