According to a Norwood quality control I interviewed back in the late 1980's, PTB stamps designated paint body and trim inspection before the front fenders were installed on the car. Most of the '68 and '69 cars have the stamps in sight but many of the 67 cars (not all) have the stamps behind the front fenders. It was about 1987 when many Norwood plant workers transferred to the Baltimore plant due to the closing of Norwood. Talked with quite a few of the Norwood workers during that time.
Jerry@CHP