ATJ1026 Nuffield Core Plug (or Welch plug).  

1. Seals the Steering Box Cross Shaft.
   (see parts numbered 5 and 23 in the parts drawing).
2. Seals the belt pulley End Nut.
   (view part numbered 36 in this illustration of a Nuffield Belt Pulley Unit).

.Fitted to tractors:

• Nuffield Universals M3, M4, MV3, PM4, DM4
• Nuffield Universal Three (3DL), Nuffield Universal Four (4DM)
• Nuffield 342, 460