And the plaster emblem pulls right out.

It's clean as a whistle.

I'm very pleased with the level of detail this rubber mold provides.

And you can see it will be larger than a regular neckerchief slide.
Tomorrow afternoon, we melt plaster.
My plan is to heat the rubber mold up on the top rack of our propane grill. Meanwhile, heat a "crucible" of pewter on a camp stove. And pour it into the hot mold, letting things gradually cool down.
The one thing I haven't figured out is how to do the hoop behind the emblem to capture the neckerchiefs. It might just be a small loop of bailing wire heated up an pressed into the molten back of the pewter.