[ngoma]

x2^

They seal not via the threads, but rather via the precisely machined mating surfaces of the nozzle, nozzle holder, ridged/cupped "washer" piece. Since you "precisely lapped all the mating surfaces," maybe a piece of dirt or foreign material is trapped on one of the sealing surfaces? Try dismantling and meticulously cleaning them, maintaining clean-room conditions during reassembly. In my experience, the ledges in the nozzle holder are the hardest to clean and verify. Using a loupe and strong light helps.