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April 1957; 1967 · SBN (boundary)
The snapshot is the April 1957 Official Airline Guide, and it shows four carriers on one field: North Central, 10 weekday departures; United, 9; Lake Central, 6; TWA, 3 — twenty-eight weekday departures in the regulated era, divided among four airlines in the proportions the Guide prints. The first jets follow a decade later: in 1967 SBN got its first scheduled jets, United's Caravelles, flown on a Chicago O'Hare – South Bend – Fort Wayne – Newark rotation and back. The month carries a spread and it is printed whole: the audit floor for this row gives January 1967; the reached sources give the year 1967 without the month. The floor's January stands as the timetable record's claim and the reached record's year-only statement stands beside it; no month is adopted beyond what is shown.
Sources: April 1957 OAG figures as carried in the airport's reference record; the same record for the 1967 Caravelle rotation.