Vin
FWIW
When your truck was new , they most often used the engine's serial number as that was the most valuable part of the vehicle then .
So , check it agaionst your title and if no match on engine nor door jamb data plate , you'll need to get a title correction , DO NOT just run down to the local PD or DMV ! .
There's a bit more to it that'll save you headaches and money too .
I and others who have done this , will be glad to explain it all later , I'm rushed right now .
BTW : once in a great while you'll find a number stamped into the top frame rail near the steering box , it'll be covered by the left inner fender piece .
I've seen it on the crossmember too , right under the cab .
These numbers may or may not match the data tag's VIN .