51 - as per Wikipedia 1989 was the last year LY was operating the B707 flights.
I will go with LY, if memory serves they were operating into ORD with an intermediate/change of gauge stop at JFK.
51. A great guess! However, it wasn't El Al.....but the flight in question did stop at a U.S. city before arriving into ORD, although the stop wasn't made at JFK.