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Big Al, I do agree with Norm that the Hydramatic is a 4 speed. The final gear is direct drive 1:1 ratio. You might have to get the car up to speed to experience the final shift. As I recall, my shop manual says that the direct drive doesn't kick in until about 75 miles per hour. Or, your tranasmission could be doing the same thing that my transmission does and it just can't shift into that gear. When my car gets up to 70 - 80 miles per hour it whines and pulsates because there is too much slop in the transmission. It can't build up enough pressure to fully engage fourth gear.
If your car has a three speed then it is the Jetaway. However, the Jetaway was not stock in the big cars. The shift indicator on my car is P N D S L R. You can identify the Slim Jim becasue the transmission case is two pieces and is split down the middle (it bolts together). The Jetaway is a one piece tranny case. Also, if you drop the inspection cover for the flywheel you will see a torque converter on a Jetaway, the Slim Jim (or Hydramatic) has no torque converter on the flywheel because it uses an internal fluid coupling.