Building a better Bb Mousetrap...

  I just finished overhauling an unusual Cabart Bb Boehm clarinet, this week, which I would guesstimate to the 1930's. Aside from an articulated C#, it has this fascinating alternate throat Bb mechanism. It took some adjusting, but produces possibly the best throat Bb I've ever heard; loud, true, and requiring no 'resonance fingerings'.
 Looking back (and doing a little research) I found that this is one of a number of successful  solutions to this problem.....

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 Here's the 'Leblanc patented' mechanism, which I've worked on at least once...

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 And here's professor Stubbins variation, which was used on Noblet, Vito, and Normandy clarinets...

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 I've played all 3, at one time or another, and all resolve the stuffiness and poor speaking quality of the throat Bb to varying degrees - with the Cabart's design being the most successful, I think, although a bit complicated to adjust.
 
 So, here's my question - Is there a reason that it isn't in common production? Is retooling that expensive or implausible? It has be cheaper than building a Full Boehm system clarinet (whose Low Eb makes for a nice alternate throat Bb) and, given the $3k+ that many performers are already paying for their instruments, it seems like a reasonable bit of lagniappe.

 I'd be interested to hear thoughts and ideas from you folks, as I'm stumped as to why some version of this mechanism isn't stock on every clarinet....