Up to 1954 has inclosed torque tube drive shaft. 1955 and later is open.
The way the axle is mounted to the springs on a closed torque tube set up puts the axle center line about 1 1/2" behind the spring center bolt. This means that when converting to an open drive rear end you can not set the spring perch, with the pilot hole in the center, right down on the spring center bolt. If you did the rear end would be about 1 1/2" ahead of where you want it.
Clear as mud I presume.
