The reason why the - S- models do not use this functionality is because they do not need it. It is a common issue with downdraft carburetors that they freeze up. The DDH/DHLA/DCOE family of carburetors do not have these issues, in contrary, they prefer colder intake air to function properly.
I'm sorry but I don't agree with you that freezing is a common fault, and I've never experienced any down-draught carburettors freezing up in the U.K. or all my years of driving. I have experienced fuel vaporisation in a carburettor owing the air being too hot, and it can even happen easily if you have the wrong in the tank for the time of year. As you know fuel companies change the volatility at different times of the year to match the seasons. The main reason for warming the intake, both air and manifold, is to keep the fuel atomised so that it turns through the 90 degrees without the fuel separating from the aerated mixture, and any that does, soon emulsifies again owing to the warmth of the airflow over it.
On the K142 the mixer was not removed, but was disconnected to remain in the open position. As Matra completely copied the Gutmann 142 conversion (Gutmann did the same thing).
Whether Matra copied Gutmann or not is a mute point, as all engine technicians know that air intake warming is rarely ever required with side-draught carburettors, and I'm sure the Matra engineers would certainly know that, but the carburettor companies which provide the equipment certainly do, and would advise any manufacturer if necessary.
For the Prep 142, of course the dealers left the mixer valve fitted but disconnected, since their was nothing to connect the vacuum valve to anyway, but a dealer never does any work that is unnecessary unless someone is paying for it! I have worked most of my life for dealers so I know what they are like. But when Matra made the 'S' there was no point in fitting items that were not required any longer.
It is notable that Matra designed and cast their inlet manifold properly for their twin side-draught carb. set up in the 142 and 'S' models, and since Matra owned Solex at the time the Murena was produced, they had their experts who would have advised or been involved in the conversion. None of the after market inlet manifolds are any good in the Murena as the left hand carburettor is far too close to the fuel tank. With many there isn't even enough room to fit a proper air filter. So unless Gutmann had their own version of the Matra inlet manifold, then I'm sorry but I don't believe Matra copied anything.
Roy
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