Somewhere about half way through 97 production the 722.6 valve body was updated. The solenoid/electronic seals were updated a few times too in an attempt to stop fluid from wicking up the wiring and ruining the ECU.
Current recommendation for "filled for life" 722.6's is fluid and filter replacement every 39k miles.
Must use MB fluid for 722.6 or 722.9 only; the expensive stuff.