When the Lear Jet entered service in 1964, departure control was not used to the rapid climb of Bill's rocket. Step climbs were the norm. A first clearance might be to 3,000 ft, then 5,000 ft, then 7,000 ft, then 9,000, then 13,000 ft before some breathing room with an unrestricted climb to flight level 230 (23,000 ft).
It was in the days before mode-C transponder reporting; ATC had to rely on the pilots word for aircraft altitude. Co-pilots became adept at anticipating (actually not being truthful). You had to tell ATC you were level at 5,000 ft just as you were climbing through 3,000 ft. It was a lucky day when a crew could manipulate an unrestricted climb to flight level 410.
Air traffic was typically the lumbering big jets and propeller driven aircraft. The system worked fine for these aircraft, not so good for the Lear Jet which had nearly a pound of thrust for every two pounds of weight. Initial rate of climb rate was typically in excess of 6,000 ft per minute...100 ft every second.
Many of the early Lear Jet pilots knew each other and developed a strategy to correct the problem. They were scattered all over the country and invited their local air-traffic-controllers for a ride in the Lears. Then they showed them the difficulty making step climbs in 2,000 ft increments.
The word got passed to Washington, procedures changed. When future clearances were issued, it was normally "cleared to 9,000 ft and expect further clearance to flight level 230". This eliminated 95% of the step climbs.