Or it might be an eight-digit serial number that indicates it was built on the th day of the year. Also in and there was a high production, "9 " is exceeded for " " or higher. After 5 came Evidently the production was high in because "6 " is exceeded. When a six-digit serial number is concerned, there is no space, because the highest number in four digits has been exceeded and so fills the space. But there are some obvious indicators—if it starts with a five, for instance.