Here are the facts as I understand them:
1. Nobody has papers of a new "Blueberry" being sold new, a Rolex service center affirmed that the 1675 never came with a blue insert, and nobody even seems to claim having seen one before 2000.
2. Very authentic-seeming blueberry inserts do exist, so the quality alone doesn't mean they're authentic. Jedly1 once took a blueberry insert (that looked good enough to him to think it might be authentic) to an RSC who confirmed that particular insert was not original, though they didn't comment on blueberries as a whole.
3. Nobody has ever seen a service receipt or a clamshell Rolex parts case for a bluebery insert. This to me is quite damning. 1 Blueberry without any provenance? Sure, I guess the service receipt got lost (and we're not talking about something from the 1960's here; the theory is that they were a service part from 2000 or so). Sure, receipts get lost. But there have been what, 100 Blueberries sold at this point? Nicely evenly spaced over time and among various sellers, conveniently. And not a
single one of these has the clamshell case with the parts number? Or the RSC receipt saying "blue insert?" It just seems mathematically implausible to me. I know where Occam's Razor falls on this issue for me.
