Who would have thought copper crush washers could be so interesting!
The aluminium 14.5mm ID ALU1403L washer is probably best used with an aluminium sump if you have concerns about galvanic corrosion.
The 16mm ID 213961 washer seems to be used for the 3.5lit Rover V8. You can also use 213961L or 213961A which is labeled a crush washer. These are all the same size and, not surprisingly, all look the same. However, they can also be 20/21mm ID!
Then we have ETC 7398 which is also 20/21 mm ID and seems to be associated with the petrol injection 3.9lit engines.
Confused? As far as part numbers are concerned it's a bit of a mess.
I think the answer is to make sure you have the same ID washer to hand as the one you took off, whatever the part number.
On the other hand RichardV6 probably has the best idea of buying a selection box.
To be on the safe side I've bought two ETC 7398 and a selection box. This should last me well past the time when I've forgotten I even owned a car!