P manufactures a 'non-electronic' antilock brake system for trucks and trailers, which purports to comply with Federal Motor Vehicle Safety Standard 121, a NHTSA regulation concerning antilock brakes. In accordance with Standard 121, antilock brakes must have an electrical circuit capable of signaling a malfunction in the brakes through an external warning light. P developed a pneumatic antilock brake system for trucks and trailers. After D amended Standard 121, P challenged the validity of the rule in federal court. The Tenth Circuit rejected P's challenge. P eventually met with D to explain the operation and features of the MSQR-5000, in an apparent attempt to persuade D that the braking system complied with the agency's safety standards. D requested that certain tests be performed on the product and that P submit the test data to the agency. Air Brake scheduled another meeting with D for this purpose on June 12, 2001. One of P's customers asked D whether a vehicle with P's brake system would comply with Standard 121, D's Acting Chief Counsel issued two opinion letters 8 days before P’s scheduled meeting with performance results, stating that the brake system would not satisfy the standard. D posted the letters on its website. D did nothing else, nor did it begin the statutory process for determining whether vehicles carrying such brakes were noncompliant or the statutory process for ordering a recall of vehicles with these brakes. P met with D 8 days later, and D asked for certain tests. P conducted the tests and send the data and asked D to post a letter from P on D’s website so that people could see P’s position on the issues. Nothing happened. P filed this action challenging the Chief Counsel's conclusion as well as the Chief Counsel's authority to issue the letter. The court did order D to take steps necessary for D to complete a review of the brakes. D reaffirmed its conclusion. The district court granted summary judgment in that interpretive letters issued by D's Acting Chief Counsel do not constitute 'final agency action' subject to judicial review under the Administrative Procedure Act. P appealed.