"Series 1" is not the original Apple Watch model. Confusingly, that's officially named "Apple Watch First Generation" and colloquially called "Series 0".
When they released the Series 2 with better processor, GPS, brighter screen and better waterproofing, they simultaneously killed the "Series 0" and launched the Series 1 as the low price model. It got the new processor but not the other improvements.
That leaves the Series 0 as the only one with the original crappy processor, and support for that has been dropped in watchOS 5.
