i believe the early thr.cable housing is measured by diameter (5 mm) not by circumference.
Steven Vanzetto would know the dimensions, he made really very gorgeous copies a few years back. i don't have a spare 5 mm cable to measure from.
they show up from time to time on ebay. if you seriously want a 5 mm cable, i would search ebay 1st thing upon awakening at least once during the day before i go to bed. at least that's my obcessive style. if one surfaces on ebay, be prepared to bid big bucks. "someone" a couple days ago on ebay, got super lucky on a complete rack of late 1-4 carbs with full set of #28 tops (no cable.) around $850 as i recall. set of #28 caps and cable was selling a couple years ago for $650 - $750.