I don't think thats isn't 1st position sprocket. I say that because it has shift gates, which are usually only cut when there's an adjacent smaller sprocket. The serations might be to improve support for a mating 12t which partly nests into it.

Also 1st position sprockets are machined from a thick blank with the spacer built in. It may be a full width spacer or partial width requiring another thin spacer behind it.

Sprockets with built on spacers (technically called hubs, just to confuse things) are more expensive to produce, so manufacturers would like to avoid needing them, or at least make them as thin as possible. But since the 1st position sprocket overhangs the end of the cassette it's necessary to have some sort of a hub, so it doesn't hang by a thin edge.