904 vs 999

Professor Fate said
I *think* the solenoid was added later on the 999s, or maybe it was dependent on the application? I've got (or rather, I had) a '78 LU 999 with no connector, pure hydraulic activation. Came out of a Fifth Avenue or Dippy, I believe.
Then I've got a '91 A999 (32RH) from a van that does have a solenoid connector that hooked into the tow/haul switch... don't know if the solenoid was dependent on the truck models or if it was phased in with the '88 computer controls for the TBI?
They did come both ways, though.
And somewhere along the line the cooler lines and fittings were upsized to 3/8, and reinforcement ribs were added to the tailhousing.

Hey, thanks! I completely forgot about those fully hydraulic lockup units. IIRC, I think that I had read on AllPar that the later solenoid was primarily just an add on to that same hydraulic system to allow for an override off of the usual activation (I.E. tow haul, or other condition programmed into the computer, maybe cold loop mode for emissions).