That pushbutton is the clutch interlock switch. Supposed to keep you from starting the engine without having the brake pedal depressed.
All the hydros are sunstrand units. The gaskets (and other parts) are available from forum sponsors, partstree.com and numerous other sources.
The hydo unit comes off the rear end by removal of the drive shaft, the suction pipe underneath, the hydro lift lines, and 4 bolts that go through the unit into the rear end.
__________________
|