NX stands for Naval Experiment, and NCC stands for Naval Contruction Contract.
That's never been proven, anywhere in Trek.
Hence the Not that it matters much in trek
. I was refering to real world.
Edit: To clarify, I don't know that the Navy actually refers to the contract as an NCC or uses it in any form of registry or identification number, other than the actual contract to build a vessel or other naval project. My understanding is that someone somewhere in trek production came up with the idea that the NCC registry could be based on the same concept, though far more literally than in real world. I've heard that it has been stated in a novel, but I don't know if the orignal idea came from a legit producer or not, and if so, if that was the original intent when TOS was first created or if it came later to explain why they may have used NCC.