IIRC Interdeck is the same paint as the Toplac but with the non slip addative mixed in. So just buy Toplac grey for a perfect match.
With regard to International not replying, just remember the experts have just spent 10 days talking to visitors to the Southgampton Boat Show and probably having a few day off or catching up with enquiries whilst at the show.
Thanks for the info. I never got a reply from International but SML paints had the International 289 number in their database and were able to supply what I needed.