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So, one of my recent eBay purchases ended up being this super clean 1970s Seiko 5 Actus SS (6106)

It’s quite nice in person, and keeps time down to the second as far as I can tell. However, I noticed something: The incredibly tiny number painted on the bottom of the Dial, reads, “6106-8930T”, while the Case-Back reads, “6106-8690” Is this a normal thing? Or has the back been replaced?