Possibly, you're not far from the truth.:)
The owner of the Zephyr trademark was Marx & Cie.(Marx & Co.), Solingen. The trademark was registered in 1900, No. 41547.
At the beginning of the 20th century, closer to WWI,
Adolph Kastor became the owner of many German and English trademarks. His brother, Nathan Kastor established a factory, the Germania Cutlery Works in Solingen. Another brother, Sigmund Kastor is a controlling director of Wade & Butcher in Sheffield during WWI specializing in straight razors.
So. May be, your razor is one of Kastor Brothers products.:)