It's likely not a "testing" issue. There is no reason for Rolex to make two versions of the same movement. Once they put the 3186 into full production, and they used up all the 3185s, all GMT-IIs and Exp-IIs were probably fitted with the 3186.
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16610 Submariner Date; D Serial
16234 DateJust SS with WG Fluted Bezel & Jubillee, White Roman Dial; F Serial
16570 Explorer II White Dial; M Serial
And Hers: 78240 Mid-Size DateJust SS with Domed Bezel & Oyster, White Roman; D Serial
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