An mkl file extension is related to the CAD6 and used for its library files.
MKL file extension - Malz++Kassner CAD library
What is mkl file? How to open mkl files?
File type specification:
The mkl file extension is associated with the CAD6, a German CAD and CAM software for Microsoft Windows operating system developed by the Malz++Kassner.
The mkl file stores data library for CAD6.