Series 1 was the 2nd year watch. Series 1 says 'SERIES 1' on the back. The older original Watch has the watch size on the back instead. The original is called Series 0, it was not the Series 1. You probably got a Series 1 as the replacement.
The first generation Apple Watch have the S1 Chip. The Series one has the S1P dual core processor. On The back of the Apple Watch, it also will say Series one.