Matthew Panzarino, reporting for TechCrunch:
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 is very similar to the connector for the Lightning connector in iPhones, as that connector currently only uses 6 of its 8 available pins. Apple recently began opening up the Lightning port for use by third parties. A source says that this port is currently for diagnostic purposes only — but that there is nothing preventing it from being a connection port for future accessories.