It's the part that threads into the adapter. I still don't know how to get said part off, it's rusted on there pretty good, cutting I feel would pinch the line.
Currently trying to do the radiator lol, the lower hose is being a pain too
Sounds like your best bet would be to use the correct hose barb fitting at the radiator and a section of hose going to the old line back to where it is not rusted. Use a mini tubing cutter. It would be best to have a compression fitting to hose barb adapter on the end of the hard line rather than putting the hose over the line with hose clamps.
I remember getting the fittings I needed from bulkpart.com