This is from the Chassis Service Manual
For data plate decoding, go here: http://www.1958buickforum.net/forums/viewtopic.php?f=8&t=254
The first digit indicates the series:
4=Special
5=Super
6=Century
7=Roadmaster
8=Limited (this was NOT in any of my books, but this is the first digit of my Limited's VIN number)
The next digit is the letter E for 1958.
The third digit identifies the plant where the car was produced:
1=Flint
2=South Gate
3=Linden
4=Kansas City
5=Wilmington
6=Atlanta
7= Framingham
8=Arlington
The remaining 6 digits are the car's individual serial number.
Example: 4E8010123 (Kevinm22's car)
4 = Special
E = 1958
8 = Arlington
I believe the serial number started at 1,000 so this was the 9,123rd car made, but I could be wrong.