Runaround Valve,
The thingo to which you refer was called a throttle bar or an idle bar. It's purpose in life was to prevent the retarding of the thrust levers too far when above FL290. It was engaged, slid into place actually, by the FE at FL290 on climb and stowed on descent below FL290.
The early JT9-3A engines operated very close to the surge line at higher altitudes and were very prone to surge without the higher idle rpm from the throttle bar. Idle acceleration was not much slower than current types. All B747s,
AFAIK, shift to flight idle with approach flap selection and maintain this higher idle speed until 5 seconds, I think, after ground mode sensed. This is purely to satisfy GA engine acceleration requirements.
Does anyone else hear the fluttering of chickens coming home to roost????