In the bus station at the Mexico border, the plural for "bus" spelled 'BUSSES' in large metal letters on the wall in the background. Until 1961, this was a preferred plural of "Bus". This story was made in 1963. It was not unusual for old signs to still be used. Therefore, the sign is not misspelled.
The killer confesses after Paul Drake says his operatives have found incriminating evidence in the killer's workplace. Paul would never have done such a thing, as private detectives are not allowed to conduct evidence searches in California; Paul and his operatives could have lost their licenses and faced criminal charges.