CAR 4b.720 in the 1950s required a FE if the work load required one. There was also a weight limit. If the gross TO weight was over 80,000 pounds a FE was required. The first DC-9-10s were delivered with the center fuel tank deactivated to keep the weight down. In 1965 the FAA changed the rule to make the requirement part of the aircraft type certification and therefore type by type. Douglas sent out some plumbing to connect the tank and new paperwork to raise the TO weight.
The 737 was certified in 1967 and some of the certification testing was to see if it was safe to fly with a two man crew. I don't know if they measured how much head turning there was but things like switch location would have be covered.