Don't see how it would be a faucet stem-
In the first picture-
The rounded over end on top doesn't have the ability to hold either a handle or a washer.
Is this piece brass for sure? Doesn't seem to have brass patina in the pictures.
I'm thinking the seat would actually be the top side of the lip on the bottom and the stem would pull up to stop the flow. This would be a course valve like used on a farm pump or such.
Cliff