Positive image on brown glass... what is this?

[agnosticnikon]

While on vacation, looking in antique stores, I came across what I thought was a glass plate negative on dark brown glass. It was cheap, so I had to buy it. Upon closer inspection, I realized it was a positive image. I've never seen one of these, and have no real experience with glass plates. The image looks to be of two women in victorian type dress. It looks like it might have been mounted in some type of case at one time. I don't believe it's a dag, as it doesn't have any of their characteristics, but I really don't know what it is, or how this was used.
Anybody have any ideas?

 

[removed account4]

hi agnosticnikon

its either a wet or dry plate ambrotype ...
they were made from the time of the civil war (wet plate ) until maybe the 20s-30s ( dry plate )

the wet plate ones are made with collodion ( spiked with other stuff ) and silver nitrate and a portable darkroom
the dry plate ones are made by using silver gelatin emulsion on glass ...
sometimes instead of glass metal was used ( ferrotype / tin type )

there are still people making images like this even today ( and even here on apug )
just google