compare '87 and '90 rear caliper

[Nick Lawrence]

I have taken apart several rear brake calipers from '90 and '91 type 44 cars and I think I have a good idea how they work.
I took apart a rear caliper from an '87 type 44 and it was different,  don't know if was assembled wrong before.
On the later caliper: 
    -remove piston by unscrewing
    -reach into cylinder and undo the snap ring and out             comes the spring and smeet metal retaining device
    -wiggle the course thread rod and out it comes with a         plate

On the earlier caliper:
    -remove the piston
    -reach into cylinder and undo snap ring
    -nothing spring loaded in here at all!
    -course threaded rod is held in very snug fitting stamped
    metal plate that has tabs to prevent the rod from turning
There appears to be a spring inside the piston on the earlier caliper.
   
The later caliper uses the internal spring under the snap ring and cover to push the threaded rod back when the park brake is released.

What causes the threaded rod to retract on the earlier calipers?
Any insight on this will be appreciated.
Nick