That is a mandrel.
The JD2bender leaves no kink,, The dies as sold by JD are sized so it won't kink reasonably thin material.
Using a mandrel in that JD bender is a waste unless you get a smaller radius die to take advantage of the mandrel. Then you need to set up a frame on the back setion of the tube to keep the mandrel from pulling through the bend along with the material. Then it will need a way to retract the mandrel from the bent material. Real mandrel benders use a big hydraulic ram. That screws into the back end of that mandrel you pictured.
Try looking on ebay.
my signature lines...not really directed at anyone in particular..
BE different....ACT normal.
No one is completely useless..They can always be used as a bad example