TWA was in B, sharing with USAir until US needed to grow into B and, eventually, C. I guess they operated the LHR route, whenever that might have been, from the old, ratty, dilapidated international terminal on the other side of 17-35.
This is all interesting history because the first 5 years of the 90's had several changes in PHL-LON authority. 1st, TWA owned the slot until they sold it to US in 1992(LGW only). But then when BA made the investment in US, they had to give it up in 1994, which is when AA got it. But then the US-BA relationship soured and they parted ways in favor of AA-BA, so the PHL-LON route went back to US where it's been ever since. Does this sound right to the history buffs?