Simpson Hall Miller & Co Repousse Jar

A 4" covered silverplate jar, repousse, from Simpson Hall Miller & Co. The inside of the lid is solid, so there was nothing protruding down into the jar. There is a white powdery substance in the bottom of the jar. Intriguing is what looks like a sealed hole on the side of the cover...we can't find this anywhere online.

Any idea what this is?

Good