There is no North Carolina State Highway 15, since that would conflict with US 15. Somebody in NCDoT has mistakenly made some NC 15 shields with the diamond shield when they meant to make US 15 shields.
About two years ago, these signs were installed at the intersection of US 15/501 and NC 54. What was funny was that sign complexes had both US 15 and NC 15 shields, one for northbound and the other for southbound. Unfortunately, I didn't get any pictures before the NCD0T realized their mistake and replaced the shields.
This week, one of the NC 15 shields reappeared. It is not quite as ironic as the previous shields, except that NCDot made the same error again.