The variable, 'reward_dur', is an editable, by default, whether you declare it in the timing script or not. It is inherited from the original ML and you cannot hide/delete it.
The value of reward_dur can be adjusted during trials, by clicking the '-' and '+' keys. The minus key decreases it by 10 (msec) and the plus key increases by 10.
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'reward_dur' will need to be defined manually in NIMH ML 3.0 and will not be added automatically to the task.