As said, nothing official. But we have plenty of other registry letter triplets in evidence, most of those beginning with N, and most of those clearly being civilian or at least nonmilitary Federation registries. So N probably stands for Federation, then - either as a "nationality" identifier, or then as sorting out government vessels from private ones. We have had a couple of N-free Federation vessels, such as the colony transport Santa Maria with BTR.Is there an official definition of what "NCC" stands for?
Foreign vessels have sometimes been identified with TLAs, too, but those appear to be the equivalent of "USS", not of "NCC" - say, IRW in front of the names of Imperial Romulan Warbirds is often seen on DS9 Okudagrams.
Could be a mouthful that different speakers abbreviate in different ways. Something like "Unitedfederationofplanets Starfleet Spaceship", lending itself to abbreviations "United StarShip" and "United Spaceship" alike.Or even "USS"?
Timo Saloniemi