Master Ultra Thin 34mm Caliber 839 or 849??

I'm quite confused by the scenario, despite having owned a MUT myself with a sapphire back, I'm trying to figure out whether some solid casebacked MUTs have caliber 839 and the rest caliber 849 or if they all house caliber 839. The reference number (145.8.79) is the same for the solid and sapphire back.

Any help on this would be greatly appreciated. I've attached two photos for reference (not my own)

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