Sorry for the misunderstanding about the action type. Yes, that is a cross bolt that is often called a Greener style lock - though several generations of German gunmakers would protest. Greener did in fact make a couple of proprietary action designs, but this looks like an Anson & Deeley lock which is a good thing on a rifle. The markings on the flats are what one would expect to see on a purpose built rifle of this period rather than something more recently created from a shotgun action. In other words, I think it is a purpose built Janssen Brothers double rifle. Assuming the barrel solder is still sound, the barrels are on face, etc you could have an interesting and useful rifle.