I had been slowly gaining confidence that the engine was all original, and had never been rebuilt. The parts, including quick to go things like plug wires, and dates were all turning up original spec. The bolts were the correct size “across the flat”, per The Maestro Harry Pellow. The only odd thing had been an oil filter canister strap.
I finally got the heads off, along with cylinders and pistons today. Not so original after all. These pistons appear very shiny compared to the rest of the engine. A friendly poster on Porsche 356 Registry forum confirmed my suspicion.
Oh well. Further justification for a complete overhaul. Also makes me feel less guilty going down the big bore kit route, instead of all original spec.