Yours would be the first of which I am aware. And to be manifest at only 6k miles means the engine must have originally been built with very with very loose piston to cylinder wall clearance, and should have been somewhat noisy from new. Piston slap is a "soft" sound generally only audible for a short while at start up until the engine fully warms through. If the noise persists with a fully warm engine, it is more likely to be related to the Centa drive coupling which are famous for various maladies and sounds most usually related to the rubber drive rollers breaking up.
And it occurs to me that if you have an early car, you may still have the original Harley-Davidson torque compensator, which needs periodic greasing or they can get pretty noisy. Most early cars have been converted to the Centa rubber roller cush drive.
