This chuck key holder fits on the back plate of most smaller lathes like the ERBA Compact 300.
The 2.5mm hole can be tapped for an M3 countersunk screw to secure the holder, so it won't come loose when you take out the chuck key.
The smaller of the two big holes is for the tool holder key.
The author remixed this model.
Added a mounting screw hole to prevent the holder from tilting.