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I'm pretty sure it's there. There are 2 different prison layouts and the lever is in different place in each of them.

In the other it's near prison office and in the other it's in this small room in the middle of corridors.

To add, the room it's in is REALLY hard to spot  in the latter layout if you're not clinging to the walls, as you can't actually see the door to enter it's room. Look in some of the rooms near the circular cell if that helps

thanks! I found it!