So here’s a Buescher tenor I’m working on at the moment (the third Buescher on the bench in the last ten days).
This has the classic Eb issue whereby the arm that actuates the Eb key has little more that a hole drilled through the arm to serve as a key barrel. Yes, you could ream it out and bush it but if there’s that much wear in it then you might as well upgrade the entire stack. But sooner or later you’re going to run into the same old problem.
In this case there’s room on the body to shift the threaded pillar (blue) down the body to a maximum of 20mm before it hits the bottom bow ring – and there still wouldn’t be an obstruction to the low C# key.
All jolly good – and now you’d have to slice the Eb lever key (red) and fit an extension – and then fit an extended lower barrel to the Eb lever key.
But then you run into a problem because the actuating pin (green) will now miss the foot of the Eb cup key (yellow). Maybe you could move the pillar down only five mm or so and still have the pin able to work the Eb cup key...but that doesn’t leave much meat on your extended barrels.
How about reversing the pin? Easy enough to do – but the pin also serves as the regulator for the lever key’s height and needs to butt up against the threaded pillar...so you’d have to fit a pin that extended through the arm.
By now it’s all getting rather complicated – and thus expensive – and for a mechanism that clearly has limited use. And that’s why the spring on the Eb cup key has been reversed and due clearance provided so that the lever key doesn’t touch the cup key foot. But the payoff for this is that there’s no chance of a leak from the Eb...at least until such times as the pad rots – and it improves the feel of the lower stack.
As for the split D system, here's a Selmer Modele 26 C melody.
I could be wrong but I think the Selmer requires you to lift and switch a finger to move between the Eb and the D. In this respect the ‘Eb round the back’ mech wins out in terms of functionality.
My Rampone bari has a similar mech but I'm pretty sure it works via split fingering. The Boosey system is different again, and I would call it semi-automatic.