Tom,
I'm sure others will be around with a difinitive answer but if the NID is similar to the Flues you should have a round "plunger" type piece in the center of the action. It holds the top lever open. It should have a hole in the middle for the spring. In operation, the spring pushes against the floor plate to push the plunger up when the lever is pressed to the right and the action opened thus locking the lever to the right until the action is closed.
Phil