Thats the one on the dash right? That one doesn't really tell you much of anything. The J is acutally a sales code for the car's bodystyle.
M= Fleetwood Sixty Special Sedan
P= Fleetwood Brougham Sedan
N= Calais Hardtop Sedan
G= Calais Coupe
B= Hardtop Sedan Deville
J= Coupe Deville
F= Deville Convertible
L= Sedan Deville
H= Fleetwood Eldorado
R= Fleetwood Seventy-Five Sedan
S= Fleetwood Seventy-Five Limousine
Z= Commercial Chassis
The Second digit is model year, and the last six are production sequence, starting with 100001.
For finding more info on your car, you can try this:
http://www.cadillaclasalleclub.org/authenticate.shtml (I just LOVE that little bold disclaimer) And I would Highly Recommend a Factory Service Manual. That is where I got the VIN info. I also have the style numbers (not exactly useful on a different model year), the algorithm for generating the code for your keys, And some ways to tell about stuff that may have been done to your engine at the factory (i.e. an asterisk in a certian spot indicates a .010 oversize bore, an L.C. indicates low compression overseas motor, and so forth)