Since we're now into September, I'm thinking 2019 is highly unlikely.
It was longer than 4 months between the announcement of the Scribe and Servio and when they shipped.
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The difference is the Prime 6 was announced already. The Scribe and Servio were announced at CES 2019 with an approx. release date (even tho a similar version of the Scribe was also announced at CED 2018). SD was quiet about it's release until it was time for pre-order. So since the Prime had already been announced with a 2019 date, I assume they will also be quiet about any release until either a pre-order date is set or it's official release.