Tim,
That cast in English scroll is going to be hard to "clean up" as the very fine details are going to get blurred when you do the basic surface polish necessary for the flat areas. If you want to try, what you're going to use are smooth Arkansas stones and lots of honing oil. A particularly useful stone is a conical that is ground to a fine tapered tip that you can get down in the graver cuts. No matter how careful you are in doing this cleaning up the end result is going to look like it had cast in engraving and will not "look right." If you're only talking about the standing breech and a few other pieces, a much, much better bet that your cleaning this up would be to have an engraver recut it for you. This is the only thing that is going to make them look right. It will cost a bit - depending on how much engraving and the detail of the pattern maybe a couple hundred - but you'll be a lot happier in the end. You should talk to your engraver first, but they are generally much happier if you do all the basic polishing - to worn 400 grit. What you're doing in this is polishing the piece as if it were plain steel that was being prepared for engraving and basically ignoring the engraving that's cast in. Those little flat areas in the pattern have to be polished anyway. Smoke print it first to get a good impression of the original engraving. To do this run the piece in a candle flame to get a nice layer of soot on it and then put a piece of scotch tape or clear, thin packaging tape on it and evenly smooth it down. Pull the tape off and put it on a piece of white paper. Instant copy with lots of detail for the engraver to work from in addition to what's left after the polishing.
Tom
