The wishbones were required because unlike a ported housing spool valve
there is no way to pipe steam from a center porting on a slide valve. If
you look at current production industrial pneumatic valves they use a
similar arrangement with the exhaust ports ie separate port to vent which
is adjacent to the cylinder operating port. This allows for a simple body
construction without internal porting. The other thing about a spool valve is that the steam pressure fills the clearance between the valve and
the housing and both centers and floats the spool in the housing. This
reduces operating forces over a very unbalanced slide valve.