My brother recently found a restaurant in my hometown that claims to have invented the bagel sandwich in 1988. I thought there was no way this was possible, but can't really find anything on the Internet about bagel sandwiches before the 1990s.
I can't tell you exactly when the bagel sandwich was invented, but at least as early as the 1950s. Page 18 of the September 8, 1950, edition of the New York Herald Tribune mentions them:
"Cream cheese, she would have you know, is as much a part of the bagel sandwich as the smoked salmon."
Page 27 of the October 25, 1960, edition of the New York Times mentions Governor Nelson Rockefeller eating one:
"Invading a supermarket near Ninety-fifth Street, the Governor dug into his pocket and bought the salami...Farther down the street...he walked into a delicatessen and emerged crunching his bagel sandwich."
So it at least predates September 1950 -- 38 years before your local restaurant claims to have invented it.
Do you mean the bagel itself or the idea of putting foods between the bagel bread?