Here's as good a history of the PHL
terminal area as I can provide:
: The core of the current terminal complex opens, consisting of what is now the US Airways ticket lobby, the Philadelphia Marketplace retail area, and Concourses A, B and C. Ticketing is on the upper level (where the Philadelphia Marketplace is now), as is a large departure lobby. On the mezzanine is a restaurant, weather station and observation deck. The concourses are two-level piers projecting onto the apron, although there are no jetbridges. Passengers walk along the upper level until they reach their gate, then take stairs down to the ground and board their aircraft. There is a 'Concourse A' for international flights, but it is little more than a single gate on the west side of the terminal. Baggage reclaim is in a small room on the lower level.
: Concourse D is constructed on the east end of the terminal, with United as the primary tenant. A long walkway connects it to the main terminal. The baggage claim area is also extended.
The international carriers - Pan Am, TWA, BOAC, Air France and Lufthansa - use the short stub of the main terminal known as Concourse A. American, TWA, Delta and Allegheny use Concourse B. Eastern, National and Northeast use Concourse C, and United, Mohawk and Northwest Orient use Concourse D. The airport will stay in this configuration until the early 1970s.
: Philadelphia begins a major renovation of the airport. The program will create self-contained 'unit terminals' along a spine road.
To free up space, the international airlines are relocated to the 'Overseas Terminal' on the east side of the airport. It's basically a giant blue hangar that was enclosed and converted to passenger operations.
: 'Unit terminals' for Concourses B, C and D are opened, and an entirely new Terminal E is built, with American, Northwest Orient and Delta as the main tenants. Concourses B, C and D are updated to include jetbridges and departure lounges at each gate. As Jetjack74 stated, construction on Terminal A (for international airlines) is halted and it sits unfinished for more than a decade... sort of like Terminal C at EWR
during the same period.
: Terminal A is finished and opened for international traffic. For some strange reason, American and Midway also move in, creating an odd split between domestic and international carriers.
: The airline shakeout after the Gulf War leads to USAIr becoming the main airline at PHL
. It takes over all of Concourse B (TWA moves to Concourse E) and Eastern/Midway's former gates on Concourse C. Concourses B, C and D are renovated heavily at the end of the decade, removing the greenish-brown paint and the giant gate numbers painted on the sides.
: Terminal F is built for US Airways Express. The new Terminal A-West follows a year later.