The Model RK23 (payload capacity: 1,500 kg) went into production in April 1956 in a minor changeover of the Model RK. It was a heavy-duty version of the Masterline pickup (released in December 1955 with a payload capacity of 750 kg), which in turn had been derived from the Toyopet Master developed for use as a taxi (released in January 1955). The changeover to the Model RK23 significantly modernized the truck's appearance. Whereas the models up to the RK featured a cab with independent front fenders and side steps, the sides of the RK23's cab became flush with the body by adopting the front body of the Master. Although both the Masterline pickup and the RK23 employed a ladder frame, the former was basically designed as a passenger car and the latter as a truck. As a result, the Masterline had a unified body with an integrated cargo bed, while the RK23 came with a separate cargo bed.

In April 1958, the RK23 was succeeded by the RK30, adopting a synchromesh transmission and switching from a floor shifter to a column shifter to accommodate three people. In July 1958, the RK35 was introduced, extending the wheelbase by 210 mm and increasing the payload capacity to 1.75 tons. The modification was in line with the reclassification of the small 4-wheeled vehicle category in June of the previous year, which expanded the maximum length from 4.3 m to 4.7 m.

In June 1959, the Toyopet truck was renamed the Stout in a naming competition participated by Toyota and sales channel employees.