Pan Am DID NOT have the monopoly on flights to West Berlin.
Under the Four Power agreements at the end of WW2, Britain, France, the US and USSR divided Germany into zones and Berlin, as the capital, was similarly divided.
As Berlin lay well within the Russian zone and Stalin wanted to do his own thing with "his" part of Germany, he only agreed to air access to Berlin from the West by means of air corridors.
These were agreed along with a restriction that the corridors could only be used by aircraft registered in the US, Britain, France and, strangely, Poland.
Air France, BEA and Pan Am all served Berlin with scheduled services up to and beyond the end of the division of Germany.
From the early 1970s BEA and later BA operated flights for Air France under code shares.
Holiday flights were provided by British charter airlines, including Dan Air, and some American financed operations such as Air Berlinn, using American registered aircraft.
Immediately prior to the end of the division of Germany, a British company called Berlin UK European was serving Berlin from various points with Jetstream 31s.
After the re-unification, when Pan Am was in demise, Delta took over the Pan Am base at Frankfurt and offered connections from both London and Frankfurt to Berlin and other European destinations to connect with trans Atlantic flights.