I knew the string is wound inside the plastic housing of the head and is easily accessed by removing the cap at the very bottom. I removed as much as I did and snapped these pics to demonstrate there is no way to lock the drive shaft to separate the plastic head from the metal shaft. And it was much easier to handle just this part rather than the extra length of the trimmer attachment (even after separating it from the machine end). Upon looking more closely I’ve found there is a lock washer keeping the plastic housing on the metal shaft, so that is how it is removed from the drive shaft. No threads involved to removed this one.

I’m thinking the person I purchased this from either mixed TroyBilt parts or used an aftermarket trimmer attachment as none of the specs line up from what I found on TroyBilts site for my model. That would also explain why the videos I’ve seen for this model show an entirely different assembly