Umbo (Otto Umbehr) was a pioneering German photographer in the 1920s. I did not know anything about him until I saw one of his prints at the Art Institute of Chicago today as part of an exhibition about shadows--

Unfortunately, they don't provide a larger version online, but the tones in the print are incredible. It's just slightly warm, with all the hallmarks of a silver chloride print. I think the German chloride paper available then was Agfa Lupex.

Here's a larger image of another print from the same series--

Here is an article about his life--