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And based on the constant change in physical format and interface I'd be surprised if anyone else ever does. Unlike hard drive physical and interface specs, Apple's SSDs seem to be different for each model and each release.

There really needs to be a market to justify the necessary investment in developing, testing, building and marketing. I just don't see a single years model and the number who would seek an upgrade would justify the expense.

I really believe that's the reason for only a 480gb model, but I could be wrong.
 
You could try and find a system pull online. Some folks don't need 512GB or 768GB and may be willing to sell theirs (or trade).
 
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