That small metal tab is an aperture-coupling prong used on older camera systems with built-in exposure meters. It mechanically linked the lens’s aperture ring to a coupling pin or meter on the camera body, so the camera could detect the selected f-stop for metering.

In other words, it was part of an early lens-to-camera communication system before modern electronic contacts. You’ll see similar couplings on some older Nikon-era designs and compatible lenses.

If Zeiss includes it on certain lenses, it’s mainly for compatibility with older camera bodies and meter finders that still use that mechanical coupling.