I believe that Melbi was a Merz & Krell brand. I’m writing about this one because the celluloid is so beautiful and the pen itself is a harmonious (if derivative) design. There can be no doubt about its debt to the Duofold. Indeed, as it’s a button filler the resemblance is complete.
I would place it around 1938-40. We don’t hear a lot about Melbi over here but like most German pens they’re very well made. Melbi also made The Transparent – the title gives away the most obvious attribute – a better-known piston filler of the thirties.
Where would you find such a pattern, and in such good condition? It’s a lovely pen.
With thanks for permission to vintagefountainpenexpert. eBay sale https://rb.gy/km1dlt
Goodwriterspens's Blog 
Collecting German fountain pens provides whole, relatively unexplored, world of manufacturers. I imagine that such an exploration would prove highly rewarding, as so many of the pens are of high quality.
Even today, there are still wonderful pen-makers in Germany, like Pelikan, Diplomat and Faber Castell. There is also Lamy, Cleo Skribent, Landauer (still making traditional piston fillers), Staedtler, and the excellent revived Kaweco. I think that they are still the premier fountain pen country in the West.
The older extinct brands are even more fascinating. We could do with a lot more research in English on them. It would be fun to start a collection!
I agree. Pre-war German pens, especially, interest me.
I have 3 of the Duofolds in this material. 2 American and 1 Canadian De Luxe. I believe they used fish scales to get the pearlescent finish. The barrels are all discoloured on the Duofolds due ,so I’ve read, to vapours from the original ink sac. Sort of a dark brown whilst the caps have retained their original colour. They are all in working order despite being almost 100 years old.
Lucky you! I thought that the Parker pattern was slightly different.