Tengu, brass jelly pans work because they leach miniscule amounts of copper into the jam. This prevents mould.
Generally brass pans do become 'coloured' with use as the fruit acids affect the surface.
That said, I have two shaped just like that but they're aluminium with the lugs cast on, while the brass ones have iron lugs riveted on.
I don't use the aluminium ones for jam, I use them for dyeing.
Fruit stains aluminium pink.
Generally the brass pans had a rolled rim round an iron hoop. That's a very thick pan you have there.
Is the body of the pan grey or dull yellow ? I can't see clearly in the photo.
cheers,
M