What is the little metal piece attached to the aperture ring in some Zeiss lenses?
What is its purpose?
It is like a little gun sight.
See for example the Planar T* 1.4 50 where the "gun sight" is below the 5,6 number.
by Paul Cezanne
It was a physical coupling for early built-in exposure meters. It hooked into a coupling pin on the camera body to tell the exposure meter what f-number the lens was set at.
It looked something like this:
Nikon F2 Photomic, 1971, from mir.com
Here's the user's guide, look for "Lens Aperture Coupling".
As for why Zeiss still has it on their lenses? I guess they don't want to leave owners of older cameras in the lurch :)