172,678 seems very high for a 550. They were in the 90,000 - 110,000 range. I don't think 150,000 has even started.
The VIN/SN is the order in which the car was built. So this could happen:
550: 100100
360: 100101
360: 100102
550: 100103
The assembly number is for the factory floor and using the above you would have:
550: 50000
360: 48700
360: 48701
550: 50001
These numbers are all just examples. So your SN equals the car # built. So SN 22,031 would mean that 22,030 cars were made before it (street & racing). For more Registry info see:
http://www.ferrarilife.com/register/