Yes it is, but there are fixed center links attached to the clasp that interface to the bracelet are polished. If that doesn't bother you then I don't see how it wouldn't work.
I think the only reason right now that they don't sell this for the 556 is that there isn't one made with the same all brushed finishing as the 556's h-links bracelet. You could get the clasp for the 104 and then brush the two center links. The clasp is also fairly large, and that might make it visually off-balance with the smaller 556.
I once met someone into watches at a coffee shop on a trip. He was like, "we should hang out sometime!" And I had to tell him I was only visiting... Sad. It's funny how a common interest can make people friends almost immediately.
I have a few coworkers that are into watches, it's fun to geek out with them from time to time.
I think they can justify the price because they're Sinn. They have a respected name, and there are many that wanted this clasp, like you and me. Also, Sinn doesn't sell direct to customer like CW (that's kind of CW's thing). And that means there have to be mark ups for the ADs to make money. Sinn products are getting more and more expensive, but it seems like their bracelets and silicone deployant clasps have always been pricy.
Because of the cost increase, It seems like the adjustable clasp will always be offered as an upgrade onto the base watch and normal bracelet. They may offer it as an option later, which will be the cheapest way to get this clasp, but I don't think they would sell their watches with this bracelet, for what they sell for now...
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