3 is a 1973 model year car that the engine went in
B is the assembly plant that the car was built in (Hamtramck)
519286 is the sequential serial number assigned to the car.
(It came from a very late build, probably June or July of 1973)
An example of the V.I.N. would be VS29H3B519286 if it actually came out of a 1973 340 Duster for argument's sake.
