Matthew Panzarino, writing for TechCrunch:

The reason I ask is that the Apple Watch has a port that the company has yet to show off. It’s being used for diagnostics and direct access to the Watch operating system, but it’s feasible that could be used to connect accessories in the future.

The port has a 6-dot brass contact array inside the groove for the ‘bottom’ strap connector slot. Several sources have confirmed its existence and placement to me.

It does make sense that the Apple Watch would have such a connection — in case there are ever software problems in the future that require a device restore. But, it’s something I never considered until now.

And as Matthew notes in the aforelinked piece, it opens up the possibilities for Apple Watch accessories — the most obvious would be a battery strap. But if Apple ever opens up the port to third parties, I’m sure some clever hardware engineers would come up with some really interesting use cases.

Update: 9 to 5 Mac has heard from their sources that these diagnostic ports will not be in the final production models of Apple Watch.