Craftsman Weedwacker

I have a weedwacker that I am trying to replace the head and put a yard gear replacement head on. I was able to remove the old head but the shaft is a solid rod with a bolt and there is no way to remove the bolt that I can see to be able to attach the new head piece. I have attached a photo and any assistance would be appreciated.

Craftsman do not make weed wackers, they get them made for them.
Some dealers have access to the codes so can decode the model number to find out which weedy you actually have.
Should you care to post it.
Funny enough very few heads will interchange from one maker to another, or even from one model to another of the same brand.
Don't know how you got the old head off if the arbour in the photo is solid with no threaded hole in it.
The slot down the side is for a locking pin which goes through the hole in the head & washer
The nut is LEFT HAND THREAD.
To get an idea, my box of arbours has 16 different Unified ( USA ) threaded arbours and 12 different metric ones
Yet I rarely find one that will actually fit
My tradies all use Drangon Fly or Tanaka heads for this reason.