One is located near the engine (usually close to the exhaust manifold) and is referred to as the "upstream" converter. The other is located further down the exhaust system, closer to the muffler, and is called the "downstream" converter.
However, it's always best to check the specific model's emissions system diagram for your particular 1997 F150 to confirm the exact number and placement of the converters. You can usually find this diagram in your owner's manual, or by searching online for your specific model.
