When I was contracting parks maintenance during the 90s I had a good worker who was not so good with his allocated line trimmer's head. Every couple of days his was worn down to next to nothing and requiring multigrips to undoo until totally wrecked. He just couldn't get the jist of holding the brush cutter just above the ground. New cap nuts were only about $10 ea when I could procure them but heads were about $60: way too dear every couple of days! The orange plastic one is an aftermarket one much the same as an original.
Not wanting to put him off I checked out a few metal lathe businesses but was not successful until I found a guy up to the challenge of my rounded design of the original. I said "a few extra should be about the same price plus a little labour and the extra materials'" he agreed and this is the resultant which was much heavier than I anticipated.
It worked a treat but the others got "lost" by other staff testing them in long grasses=funny about that. I borrowed a metal dectector at one location and concluded the guys wanted them more than me.
The rare earth magnets are to display/prove it is made of steel.
So if you wear out your whipper snipper's wing nuts or caps try this.