PART NO.  SPL.PAGE  DESCRIPTION                                                            
2K5380    15.F.1    Plug -drain (gearbox) - Com.(E)GD101 (late 1500) -(was 6K499) -
                     -Believe this to be a typo in the 1500 SPL - should be 2K5830
2K5830    16.F.1    Plug -drain -(GB casing assy, GB assy, 1600 and 1600-MK-II)

2K5830 328-055

2K5830X 328-056

The smaller 6K499 plug has 1/2-14 BSPT thread.
The larger 2K5830 plug has 5/8-14 BSPT thread.

For more information oni this part, see Rear Axle Trch article RA-117.