Sometimes you buy trimmer line that comes with a cutter mounted on the package.   They ether run their life on the package or fall off the day you get them home.

Simple bracket to mount the cutter on a surface.  They seam to have not changed much over the years.  One I have is new and the other over 5 years old.

Two to choose from.  The only difference is the width.  20mm or 28mm wide.

Assembly:
Open the trimmer, place the lever through first and then the base should fit right in.  

Support from the plate for the indent at the base.