The mystery surrounding that Rolex watch was finally resolved in today's season finale of Hawkeye, and it was exactly what many of you expected/hoped for. It turns out the reason Clint Barton was so keen to retrieve that timepiece was because it would have compromised his wife, Laura, a former Agent of S.H.I.E.L.D.
Before becoming an Avenger, Clint also worked for Nick Fury's spy organisation and that's obviously how these two met (and why Laura was so close to Natasha Romanoff). It's revealed that the watch turned up at another black market auction in New York City, and it's when the hero's wife turns it over that the S.H.I.E.L.D. logo can be seen along with a designation: 19.
In the comic books, Barbara "Bobbi" Morse, a.k.a. Mockingbird, was introduced as Agent 19. That character later married Hawkeye, so it seems Laura was once the MCU's Mockingbird! This makes sense on a number of levels, and we're guessing the reason Bobbi became "Laura" is to hide her identity and to keep her and Clint's children safe.
The bigger news, though, is this arguably confirms that Agents of S.H.I.E.L.D. is not canon in the Marvel Cinematic Universe. There was nothing in today's season finale of Hawkeye that linked The Kingpin to Daredevil, and it's become apparent that Kevin Feige and company won't be beholden to creative decisions made by Marvel Television (hence why the Darkhold was rebooted for WandaVision).
Agents of S.H.I.E.L.D. obviously happened on another Earth in the Multiverse, but in the MCU, Linda Cardellini plays Mockingbird, not Adrianne Palicki.
This isn't really something to get upset over as it doesn't diminish the importance of that ABC series. What it does mean is that anyone hoping the likes of Agent Coulson, Quake, or Melinda May will appear in an MCU movie or TV show with their history intact are likely to be disappointed.