Please identify this old brass lens

[secludedsea]

I recently picked this up, but don't have a date for it. My first thought is the late 19th early 20thC Dr Krugener folding cameras with 'Delta' inscribed on the lens plate, but the one I have looks like a slightly different inscription, and features a sort of Art Nouveau styled flourish. I see no serials or dates on my copy either. All help appreciated. Iris opens up beyond f9 to about f4.5 I would reckon.