After cutting both threads, turning down the center part.
I could buy some thread cutter heads with missing parts on Ebay Germany.
These heads are very comfortable to use and with two of them one is covering 1/4" to 1.1/2" thread cutters tails. One has a MT2 tapper, the other an MT4.
Those parts had to be made. The threaded lead screw as well as the jaws must be right and left threaded so that the jaws clamp the thread cutter tail. The parts on the right are original parts
Milling the lead screw stopper