I'm trying to determine what airport had the FAA assigned identifier of LXB back in 1968. Many airfield had their identifier changed in the 1970s and some 1068 airfields no longer exist. Perhaps old aviation publications might have that kind of information.
There are two series that come to mind that might have what you are looking for. Both are with the Cartographic Branch of the National Archives at College Park. You can contact them at carto@nara.gov.
Maps Showing Airfield and Seaplane Stations of the World, 1968-1973
The second series is a bit further back, but if the change happened between the two series, at least you'll know where the airfield once was.
Best of luck with your search!