It was in the sight glass fitting, I used a extractor tool that got the nub out of the fitting. The threads in the cooper tank just kind of fell out in little bits.
My dad's tap & die set handle isn't for the round dies but for the hex kind... now I need to find a round die handle. Knowing my luck I'll have to buy a die set for a handle.
@Paula - thank your for popping in and helping!