Just take a mm. scale and measure it. That would be helpful. If all you have is the number on the strap and can't measure it contact VC in London.
I'm sure they can help.
It looks like the buckle end is a little bit smaller than the lug end. So without anything to comare it to, it coule be 18/18, 17or16/14 or 20/18.
I do not have my Contemporaine with me so I can't measure it til i get home on Snday. But please clarify weather it is for a Traditionelle or Contemporaine.