Baader T-2 quick coupler, Zeiss micro bayonet / T-2i (old design).
The adapter enables the Zeiss Micro bayonet to be connected to the
T2 male thread of adapters. The Zeiss Micro Bayonet is under
other things used on large field binoculars.
With the counterpart, the adapter with the Zeiss Micro Bayonet on the outside
on the outside of the T2 is the T2 quick-change system.