pknauth: I don't think the 170 is shorthand for frame length as my pickup is a long bed. You are right about the 170 having to have been written on the firewall before the cab was put on the chassis. You could see it when the cab was on the frame with the engine, but not very well. I really thing a lot of these numbers were randon like mentioned above as inspection numbers.
I have seen two pickups, same year built in different plants with many differences between the two. One that pops to mind clutch head screws on one and phillips head on the other. Different plants had different markings.
