Interesting from 1959 onward, excluded 1975, there was 3 years between code cycles. For some reason 1971 went 4 years to 1975 cycle
It would be curious to see an old letter of memo stating the reason, in the pre print its called the 1974 NEC and there was a NFPA annual meeting meeting scheduled to adopt it in Miami. My guess is the energy crisis resulting from the Yom Kippur War along with the ongoing Vietnam war likely delayed the code release. Thats also why the 1942 & 1943 codes are 'supplements' in a 1940 book with some of the requirements waived 'for the duration of the emergency'
