16R would be the most likely chassis from my perspective as well, based on your description.
There were some other FINA liveried GTRs used in promotional displays, but they were the slightly different than the ones which were raced, and commonly referred to as the FINA Presentation livery. The reference shots I have of them either did not have a race number affixed or wore the number "1" which has no relation to any real racing event with that livery.
In my image archive I have what appear to be four different display venues featuring what should in all likelyhood be #16R wearing race number 38, so it seems that it was the other one used for some of the promotional work. Perhaps one of these images below matches the setting you recall?
The final pairing has "Mondial" in the filename, which I believe would refer to the Paris motorshow.
Anyway - welcome to the forum Sami. Great to have another McLaren enthusiast find their way here.
This is without a doubt the most knowledgeable and passionate group you'll find on the web associated with the F1 so you should certainly enjoy your stay. If there is something we don't know about the cars, then it's generally still a mystery or just not public knowledge. We do learn new things from time to time and this place still manages to hold my interest after almost 6 years or so.
>8^)
ER