Bob and all..
Finally was able to reach "Richard" at Acme Air Parts. here is the link
Motorhome Dash AC and Heat systems
He explained that part 4100173 was no longer available and with two other available parts could be inserted into the existing MH heater control. These are 4100265 by-pass fitting and 570-215 cable control water valve. Both are available and listed on the Acme web site above.
Richard went on to explain that there was no easy way to get at the existing valve (WHAT A SURPRISE!!) and suggested only replacing the valve if indeed hot air flow was detected when cool was selected and the aircon engaged. so as I'm still not able to test the recently replaced aircon circuit, I'll wait as replacing the valve where it is snugly located in the passenger side cowl means skinned knuckles and a fairly salty vocabulary! anyway..thanks for your help on this..I'll let you know what happens. In the meantime, thought you might like a preview of the next generation Airstream Motorhome!