How do you get all seven premium knives?

How do you get all seven premium knives? - Thank You Signage

On the premium calendar it says you get five knives, but there are seven knives marked as premium content on battlelog. Do you get the other two randomly by opening battlepacks while being a premium member, or is there some other way to acquire them?

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Opening 5x PREMIUM KNIFE Cases... (HELLCASE)

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