Thanks for the help - as soon as I saw it written out, it was a head slap moment - how could I have forgotten that Italianate name?
The one thing that I recall about the mechanism was the "slop" in the portion mounted on the barrel. I never got to handle it, as I said, but I saw the clarinet majors do so, and the thing rattled as it was being handed from music student to music student. That much play (which could be seen - it was that loose) could not have made for a precise, crisp action when toggling the vent open and shut.
Of course, the Marchi vent is mounted on the barrel, not on the body. That alone mandated some sort of "flexible" connection that could be adjusted during the tuning process. If I was doing the design, I would have put some sort of spring follower in the mechanism to take up the slack once it was on the operating plate on the upper joint. This would have taken out the slop, while not affecting the operation.
But, I'm just a stupid Bavarian/Prussian hybrid, not an artistic Frenchman, Walloon or Italian. My father was more likely to take a hammer to one of his myriad of innovations than a screwdriver, and I still have trouble resisting the temptation myself. Hell, I'm the guy who used to whale away at the final drive connections on my tank with a hammer (a 15 pound lead makeup hammer, mind you - something that the Army didn't see fit to include in my driver's tool kit, and which I had to have my practical-minded Prussian mother send me by mail once I noted the deficiency), so I'm not the best judge of delicate mechanisms.
Personally, I think that the altissimo register on all of the members of the clarinet family is overused. I'd prefer hearing the same thing performed on a smaller clarinet (within limits, of course - the Ab horn is more of a curiosity than a musical instrument) than on a larger one taken up an octave.
An excellent example of this nonsense is the "Spanish Panic" number in the Mary Rodgers' musical Once Upon A Mattress, where the "composer" (use of quotes there intentional; see below) set the bass clarinet playing in the altissimo while the clarinet plays in the low clarinet and chalmeau registers. Apparently, she didn't bother to look at the range charts provided to her by her teacher; also see below_.
(My pianist at the time, who had been her theory teacher when she was writing her one noted musical theater effort, said that she was pretty stupid when it came to composition. He didn't need to convince me, I tell you what. Ultimately, after playing all of those high Es and Fs on a horn that needed adjustment, I just gave up, and the next time that I did it, I just brought my soprano, played everything in that one number down an octave, and no one was the wiser. One more box to carry, but a hell of a lot easier and more melodic as well.)