To put the replicant timelines into quick perspective, in the original film, Harrison Ford's Deckard was hunting Nexus-6 models. In 2049, Dave Bautista's character is a Nexus-8, and Ryan Gosling's K is a Nexus-9. And, obviously, the androids evolve with each new model number. Nexus 6s had a four-year lifespan. Nexus-8s, like Bautista's Sapper Morton, can live much longer, and can apparently visibly age.
K's model, the Nexus-9s, are similar to the 8s, but are apparently much, much more subservient. In the "2036: Nexus Dawn" short, Jared Leto's Niander Wallace introduces the first Nexus-9, who's happy enough to stick a glass shard in his own jugular at a word from Wallace. They're pretty foolproof, is what we're saying.
So what about the missing model? Where are the Nexus-7s? As far as we know, neither film explicitly mentions a Nexus-7, but 2049 left an Easter egg that reveals Rachael (Sean Young) from the original film to be one of those—possibly the only—elusive Nexus-7s. When K is hunting down the serial number from the bones he found buried under that tree, the serial number begins with N7. It's possible that's short for Nexus-7. The bones, of course, turned out to belong to Rachael, which would make her a Nexus-7 (if that's what the serial number implies, anyway).
And that alone opens up a whole new can of worms. Could all the Nexus-7s reproduce? Was Rachael the only Nexus-7, or a special version? And if Deckard's a replicant, is he a Nexus-7 as well? Yeah, have fun answering those.