I asked Wayne Stahnke today if he had ever seen Reisner valves with 
return springs.  His answer was "yes and no"!

To check-out the breadboard electronics for the new IMI Cassette 
Converter he bought a set of horseshoe chest valves from Reisner.  
When he asked if the valve had a return spring, he was told that it 
did not, but they could send him a set of return springs which he 
could install himself.  But the springs received from Reisner had 
such inconsistent forces that he threw them out and had a new set 
made to his own specification. 

Since Reisner offered springs I assume that they sometimes supplied 
valves with return springs installed, which explain why "some do and 
some don't" contain springs.

The Reisner horseshoe valves (with proper springs) worked okay in 
the Ampico A piano, but were bulky and required a large shroud 
enclosing the valve assembly in order to muffle the clicking noise.  
That's why Stahnke designed his own valve: he wanted it quiet and 
small.  The resulting valve was less than 3/4-inch square by about 
2-1/2 inches long, and quiet enough that no muffling of any kind was 
needed.  He thinks that he measured the "on" and "off" delays as 6 
milliseconds each.  That's pretty speedy!

Thanks for your offer to supply me with some different valve designs 
for comparative testing.  When I'm ready, in one or two months, I 
plan to "put out the call" to everyone.

-- Robbie Rhodes