Some Petzvals sold with projectors etc have a slot for water house stops, I have one. This was because the R-R's largely killed off sales of Petzvals as taking lenses and often remaining stocks were bought by manufacturers of Lantern slde projectors.

Presumably it's a French lens and I'm not familiar with French manufacturers, I bought a small R-R at a antique/Flea market two weeks ago, made in Paris but can't decipher the makers name.

Ian