I dont believe cars were built in strict numerical order. While the dual serial number cars were likely built before the single serial numbers, I can show you much higher numbers that are in fact, dual serial numbers....from memory they stretch into the 600s...I dont think they did dual serial numbers, stopped and then restarted the practice.
http://www.thecoralsnake.com/dualserial
http://www.thecoralsnake.com/dualserial