The attached photos are supposedly showing the change in the extension housing when the 4EAT Phase II was modified to "direct control", which included the relocation of the AWD solenoid and the "reversal" of the control programming. The later tail section appears to have different "ribbing".
I'm not sure if the change was coincident with the introduction of MY2004 cars, or actually effective with MY2005 for the Outback, although I guess that's moot at this time. (Subaru often phases in such changes, with some models getting the newer version earlier.)
The valve bodies certainly would not be interchangeable.
ntippet's waveform explanation of duty cycle % is excellent!
With the 4EAT that has the multi-plate clutch transfer system, the rear VSS is not necessary as far as rear wheel spin is concerned because the rear drive (shaft) cannot turn faster than the drive to the front differential -- it's simply impossible if nothing's broken.
This does not take into account what one wheelmight do as the front and rear differentials will allow one wheel to speed up while the opposite wheel slows down. However, the VSSs are monitoring the drive shaft speeds, not individual wheel speeds.
The rear VSS is used as a reference; the TCM compares the front and rear VSS signals and should adjust the duty cycle if the front VSS is showing a higher speed than the rear VSS. The basic logic of the AWD system is that the two drives be turning at essentially the same speed and if they're not, it increases the clutch pressure to effectively lock the drives together.
Note: The TCU also uses "torque" as an input to it's duty cycle control. In the FreeSSM post that ntippet linked, the third chart shows the front wheel speed (VSS) ramping up while the rear wheel speed (VSS)remains low, yet the duty cycle does not peak out. However, in this particular case, as I recall, the front wheels were intentionally allowed to spin and with the rear drive effectively disconnected, there was little torque being developed in the drive train. Later on we examined the impact of torque as represented by the difference ("delta") between engine rpm and torque converter turbine rpm. This appeared to be a significant, if not overriding determinant of duty cycle.