Are there old FAA records that list airfield identifiers back in 1968? I know where LAX is but where was LXB? Somewhere near Sharon, PA.

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.

Airfield Maps, 1944-1949

Best of luck with your search!