Schilke D5.1 Mouthpieces

[rmb796]

Hi Everyone,
I have two D5.1 mouthpieces. One has a double "ring" above the shaft and the other has a single "ring".
I believe the double ring may have been an earlier production. Does anyone know the history or the difference in these mouthpieces?
To me the double ring is better in the upper register and the single ring is more open in the lower register.
Maybe it is just my imagination.

Thanks
Randy