That machined step on an Ai lens matches up with a piece on F3 generation and newer Nikons to indicate the f-stop of the lens. A pre-AI lens can't be mounted on these cameras unless either 1) your camera has a little button that lets you swing the tab out of the way - if so, you can mount a Pre-AI lens and use stop-down metering. or 2) Your Pre-AI has been "Ai'd " which means the original smooth F-stop ring has been replaced be a new F-Stop ring with the appropriate Ai notch cut out. Nikon used to provide Ai kits for doing this. If your lens has been Ai'd, then you can use it on any Nikon SLR.
If your lens has not been Ai'd, unfortunately, there aren't many kits left. However, there are a few people out there who will machine the cut-out in your existing f-stop ring.