Some early-production 1960-model Slant-6s came with a bolt, but it was eliminated soon after SOP (with no other related changes) because it is not at all necessary. The vibration damper is an interference fit (press fit) on the crankshaft; it won't be any more secure with a bolt or less secure without one. Sometimes it's convenient to have a bolt there for easy hand-rotation of the engine, but other than that, no reason to have it.
