Adrian
The part number given to you is for the gearbox spacer, which is inboard of the gearbox. This is the 2mm thick spacer in the original exploded drawing.
This is not either of the spacer behind the clutch basket shown in your latest photo. The two spacers shown in your photo are the larger spacer 0258.16.840 which is 20.3 x 25 x 5 and the other with red arrow is a 0.5mm washer, which goes against the bearing, followed by the 5mm spacer then the drum. The 2mm thick spacer is the other side of the bearing inside the cases.
Not sure what the 0.5mm spacer was intended for and strangely doesn't appear in the parts book, added to that some of the spacers are 5.5mm that I have found in engines as well. The 0.5mm spacer washer is in fact 400.49.210 which is normally found in behind the output gearbox bearing as part of the sealing thrower system. Whether the factory just put them in for spacing reasons, due to some more dodgy tolerancing problems?
As far as I am concerned multiple spacers is not good policy, so I tend to do away with the shim washer and use just one spacer. As long as there is clearance behind the clutch drum and the crankcase wall, the primaries line up and the basket clears the flywheel then all is fine.
If you don't need the 2mm spacer and need anything else instead, contact me and we will work it out.
Best Wishes Nigel