Up through 1968 equipment grounding conductors did not require a 'listed'connector or equivalent insulated covering (IMHO).
The 1968 language: 'All splices and joints and the free ends of conductors shall be covered with an insulation equivalent to that of the conductors.'
Since EGC were (and are) bare, an 'equivalent insulation' would be nothing.
In 1971 the language changed to add: 'or with an insulating device suitable for the purpose.'
In my opinion, this new language in 1971 brought about the requirement for some sort of connector for EGC's.
