This subpart applies to manufacturers of passenger cars and trucks, buses, and multipurpose passenger vehicles with a GVWR of 4,536 kg or less. However, this subpart does not apply to any manufacturers whose production consists exclusively of walk-in vans, vehicles designed to be sold exclusively to the U.S. Postal Service, vehicles manufactured in two or more stages, and vehicles that are altered after previously having been certified in accordance with part 567 of this chapter. In addition, this subpart does not apply to manufacturers that produce fewer than 5,000 vehicles annually for sale in the United States.