There are several components that there is leeway given for safety reasons. Look a replacement rubber brake hose lines, look at the crimp on the connector ends, Headlights, Ball joints and others. Believe it has something to to with DOT and products driven on the street.
Having shows and judging cars so that people will pay is a balancing act. If you make the rules too tight only the rich can restore or own them. Make them too loose and you will not get quality cars.
Paying $600.00 for an NOS radiator cap is well on the way of being out of the reach of the average Joe.
Sorry, sometimes I go off the deep end. I guess may age is showing.