Colorful diamond frame

Again an unknown diamond frame. Look at the bracket and some nice details, like the upper rear mudguard support (3rd picture). Never seen this before.
On top of the handlebars is a number engraved, looks like 9073 or something in that direction. It is a shame both hubs and wheels seem to have been changed at some moment in the past 100 years. They also made a total mess of the crank axle, but cranks and chainring look original. 

It belongs to an German collector, but he doesn't know the country where it was made or first sold. It might be German, but I am not sure at all.  Someone suggested it might be a Dürkopp. Anyone can confirm this??
More pictures in three links, if you are interested:
You recognise any details? Please let me know!