That extra tab is the Maximum Aperture Indexing Lever, also called the Lens Speed Indexing Lever. Its job is to tell certain Nikon F-mount camera bodies the lens’s maximum aperture.

It was used on some AI-S type lenses and supported by specific film bodies, notably the Nikon FA and F4, to improve metering accuracy, especially for automatic multi-pattern metering.

So it is not the aperture-actuating lever that physically stops the lens down. Instead, it is an information linkage used by compatible bodies to identify lens speed.