Circa 1850s Buffet Crampon enhanced Boehm A with integrated barrel!

[Dibbs]

Have you guys done a test and seen if the donut key actually has any noticeable impact on intonation versus the non-donut Eb fingering?

(I haven't, but I can't imagine it does much, or else it would be ubiquitous today, right?)

It's just a different way to implement the 7th ring.  No better or worse than the usual way. 

[modernicus]

Thanks for showing this one! What is the purpose of the "donut" key vs. a normal ring key?

Cross fingered Eb/Bb in combination with the extra ring.

OK, yep, that works!

[Dibbs]

It's just a different way to implement the 7th ring.  No better or worse than the usual way.

I take that back.  There is a small advantage in that the D#/A# cross key is excessively high with the normal full boehm arrangement because it lies on top of the extra vent cup.  With the doughnut that key can be at the normal height.