It is difficult to keep commenting on the relentless onward march of the successive Dura-Ace mechanical derailleurs of the modern age. They get more speeds (now 12), they get lighter (now 162g), they get marginally longer pulley cages (now 59mm). But that was also the basic story with the previous generation... and the generation before that...
In response to this problem the earnest, Dutch, Shimano Europe marcoms people at trade shows, like Eurobike, had to add ever more spurious embellishments to their sales patter.
For the Shimano Dura-Ace (9000) the theme was traditional Japanese crafts honed over millennia. On the one hand you had Shimano's home town of Osaka as the seat of the sacred Japanese art of sword and knife making. All hand-fettled perfection, swooping curves, precisely defined edges and smooth, steely, polished finishes. On the other hand you have Japanese lacquerware with its signature stunning, high-gloss, black surfaces faultlessly varnished with the deadly poisonous sap of the Urushi tree. These two themes were skillfully melded together in the styling of the humble Shimano Dura-Ace (9000) derailleur - and it actually worked rather well.
What didn't work quite as well was having a, bright-eyed, corn-fed, alarmingly enthusiastic, mountain-biker-dude from Eindhoven trying to claim that, as a Shimano employee, these crafts were an integral part of his 'cultural DNA'. It was a bit like discovering that your sushi was topped with an impeccably cut, perfectly fresh, relentlessly raw slice of... Edam.
But most of all the Shimano Dura-Ace (9000) derailleur marked a point of transition in Dura-Ace history. Up to Dura-Ace (7800) the groupset was mechanical. Dura-Ace (7900) was a mechanical groupset with a Di2 electronic option. From Dura-Ace (9000) onwards the groupset was fundamentally a Di2 electronic system - but with an increasingly marginal mechanical option.
And this explains many of the styling choices. The half-polished-half-black look is designed to suit the bulky b-knuckle of the Di2 derailleur. The mechanical derailleur, almost grudgingly, gets it as a hand-me-down.
Finally, I would note that the pulley cage on the mechanical Dura-Ace (9000 SS) is 3mm longer than the cage on the electronic Dura-Ace (9070). It also has a larger pusher plate at the guide pulley. Strange, huh?
Browse associated documents.