OK, so I took the carb off and apart to see the problem. The brass insert is really the captured soft washer and the threads in the alum carb body are OK.
The jet, however is not. It is new from Sudco, so it should be jen-u-wine Keihin. On close examination, there is a small crack all around the second thread from the sealing taper. So the jet had a defect in the material or thread cut and one more attempt at tightening would have resulted in the threaded portion shearing off in the carb body. Popped in a 162 in the interim, after making sure it's threads looked OK.
Now I've seen everything. (almost).
B