I suspect your inlays are German silver, or coin silver, instead of pewter. The formula for German/nickel silver, and coin silver, surly have changed over the years, making some old inlays more likely to show age than others. Inlays can be easily be reproduced by removing what is left of he old, making a rubbing of the existing inlet, gluing it to the nickel silver, and cutting it out with a jewelers saw. Trying to pour an inlay is most likely a formula for disaster.
Hungry Horse