The Series 1 is the original Apple Watch but with the processor from the Series 2. It's a different device because of that, and lots of us call the original a Series 0. However, go on Gumtree and you'll see plenty of peppy claiming they're selling a Series 1 when it's actually an Original Apple Watch.

To make matters worse, the SoC in the original Apple Watch is called the S1, and the chip in the Series 1 and Series 2 Apple Watches is called the S2, making the numbering more confusing and preventing us from shortening anything to S1/S2.

Thanks Apple!