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!

The processor in the first gen Apple Watch is so slow that the Series 1 actually included a faster processor :)

https://en.wikipedia.org/wiki/Apple_Watch#Second_generation

The good news is that Apple Watch appears to hold its value like other Apple products. I just spotted a recent one on eBay that's exactly the same as mine which sold for $135.