Yes, the numbers are known. It’s unlikely you have the last KR built.
“518” is a reference to the total number of convertible KRs built.
A picture would be most helpful, I’m surmising from your post, this was a sign made by a previous owner?
Because, cars were not always built in strict numerical order, it’s probably better to describe a car by its Shelby serial number. In this case your Shelby number would need to be around 4400, in order to be a contender for the last convertible.