Locking Ring, part number 2810 for the Lido 2, coarse thread.
This part fits the Lido 2 ring burr carrier (2809) & adjustment ring (2811).
This will not work with fine thread Lido E or Lido ET unless you replace the whole adjustment stack (a full adjustment stack consists of 3 parts: Lido 2 uses: 2809, 2810, and 2811 / Lido 3 uses: 3809, 3810, and 3811). / Lido E & ET use 4809, 4810 & 4811 - gray OR 5809, 5810 & 5811 for black)
This replacement part is NOT returnable - be sure it's what you need before you purchase.