Wow that is cool. I wasn't expecting much when you said it was plaster, but then I scrolled down and did a double-take. That is really neat. The molded plaster frames are referred to as "Gesso" frames. Gesso was developed in order to save time so that wooden designs did not have to be carved in architectural features, frames etc. The technique dates back to the 18th century, so dating your mirror would be very difficult without a maker's mark.