Replacement focus tab for the Leica 28mm Elmarit v3 (11804).
Affixes to the lens with an M1.6 nut (not supplied) glued into the back of the focus tab. An M1.6x3mm screw then screws into the tab from inside the lens (the head of the screw may need to be filed down to fit).
See these threads here for details;
RFF;
LUF
To fit lens diameter of 52.0 mm