That looks like it might be a fitting for one of the cylindrical block heaters that Honda uses. I wonder if anyone actually installed one on an NSX? If you plan to pop the cap off you better have the cooling system drained because it is pretty much going to empty out on top of you if that is the block heater plug.
I have never seen a genuine NSX block heater; but, if it is like the other Honda / Acura heaters of that cylindrical style, it actually threads into the hole and it seals with a metal crush washer much like on the oil drain plug. That cap obviously does not thread in so I don't know how it is sealed.
The o-ring seems to be this one: 91301-PR7-A00. According to the parts catalog it is sold separately.
Be prepared to have a free shower even if you drain the coolant.