The lock rings are indeed to prevent the pressure valve holder from unscrewing. Torque to 25 nM twice, then to 30 nM without loosening or they will leak.
Leaking seals will cause severe injector knock at idle with very rough running that gets much better at higher speed, no smoke. Loosening the injector nuts one at a time will isolate the leaking one, retorque.
Use antisieze on the lock ring screws, they get stuck if you don't.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!