This is not as straight forward as it should be !! While I have written this for Australain Mokes, it may apply to the Portugese and Cagiva models, I have no experience with them, so I can say for sure.
From 1968, Mokes were fitted with a Expansion Tank;-
(yes, the book I got this from is old !)
and as such therein brings about confusion as to what Caps to buy. Car needs to be taken here as the wrong cap can cause a server over-presuration of the system, that can lead to a dangerous situation occurring.
Caps
From Left to Right - Long Neck type, Short Neck type and Blanking Cap
Underneath they look like
Long, Short or Blanking?
If you have an Expansion Tank (or sometimes called a Header or Over-flow Tank), then on the Radiator itself, you should fit a Blanking Cap.
Now, I did check which Cap would suit the Expansion Tank. It should be a Short Neck type, however, it will only take a moment to check
Here I'm actually measuring a Radiator Neck, however the one on the Expansion Tank is almost the same. This one is 20 mm deep. Other types could be 25 mm deep.
For 20 mm Neck fit a Short Neck type, CPC Part No. 508-13
For 25 mm Neck fit a Long Neck type, CPC Part No. 517-13
I have a Cap, how do I check it?
If you have a cap and you don't know if it's a Long or Short neck type, you can measure them
Short Neck type will be ~23 mm
Long Neck type will be ~27 mm
You can see from the top photo that there isn't much between them, I know I have to measure them to be sure!
Part Numbers
Blanking Cap - CPC P/N 2001
Short Neck Cap - CPC P/N 508-13
Long Neck Cap - CPC P/N 517-13
I have quoted CPC Brand Parts here as that's what most part's suppliers will likely have and they are Australian Made
If anyone else has part numbers in other brands, then please post them up.