Okay so I've got down to a lot more. I've laid the paint stripper on A few more times, once leaving it overnight wrapped in plastic and this is as far down as I could get things. Coarse grit sand paper and some work with a sanding wheel (which worked well untill I clipped the stay with the metal wheel, I went back to hand sanding after that haha)
It looks to me like what some previous posters had thought; a blobby brass repair of a seat stay cap.


You can see the knick i put in it with the grinding wheel a few cm below the lower blob


Front view I can see a the blobs of brass tracing the lower portion of the cap


Little line goes part way around the back but i dont see any cracks.


View from left