I think you are right. I keep finding different ways they did it, so sounds like the 1800s was a busy period in the development of mirror technology!

Pretty sure they just poured silver nitrate over glass. You can still buy kits to do that to re-silver old mirrors for the original look. From what I can find, the layered ones were older, and they used tin and mercury which made breaking a mirror a rather unlucky event.

Still better than the Gwyneth Paltrow Oreos. Would not recommend.