I had a handful of trigger guards in the rough (now down to only one) that are from one of the Spanish SxS's. I think Uggies but I'm not sure. I use(d) them for custom projects.

Anyway, they are not castings. They are stamped from sheet stock, bent to shape over a form.
The spur is welded on as is the threaded stud.
The discoloration from the weld is still on the steel.
I'd guess an induction(?) type weld as the discoloration doesn't spread far from the point of weld. My knowledge of welding is severely limited so excuse if I use the wrong terms here but you get the idea.

The area on the sides where the spur meets the bow and the inside of the bow itself is coarse belt or drum polished.
But other than that it is still in it's stamped out steel finish.

As far as these guards are concerned, no castings involved and I wouldn't want to polish out one that was if I could avoid it.