There are two standards for airport identifiers, ICAO and IATA. The ATA codes are the three letter codes you're used to seeing on bag tags. The ICAO codes are four letter. For most ICAO codes the first letter is a region, the second letter is the country and the last two letters are the airport. So E - Northern Europe, H - Holland, AM is Amsterdam Schiphol. R - western Pacific, K - Korea SI is the Seoul Inchon airport. Works good as long as you have less then 676 airports in your country. In the ICAO system the U.S. is it's own region so there is no second letter country code. So the major airports in the US have the same ID just with a K in front for ICAO purposes. Alaska and Hawaii are in the P region for ICAO and have A and H as their second letter. So we get PANC and ANC for Anchorage but PASI and SIT for Sitka. Same thing PHNL and HNL for Honolulu but PHOG and OGG for Maui.

As for the OP's question, I'd stick with Covington. The field was lobbied for by a group of Kentucky folks during WWII. If it was going to be the Greater Cincinnati Vicinity airport, why didn't they just make it GCV?

It used to be fun to get pilots to look for the Cincinnati airport charts in the government NOS chart books. Because Ohio and Kentucky were in different books, they'd pull their hair since all they could find was Lunken - LUK.