Zenjian was sold by the original Ludwig company and by Ludwig once Conn bought the company in 1930. Leedy also lists them in the 30s and 40s. They are not listed in the Leedy and Ludwig catalog or any Ludwig catalog once W F Ludwig bought back the Ludwig name. The WFL company never sold them. Although Ludwig did continue to sell "imported" cymbals into the early 60s. These might have been Zenjians or some other brand. Other than that, I don't know any details about them. Some people theorize that they were made by Zildjian as a second line, but given that at least some Zenjians are Italian made, that probably isn't true. According to George Lawrence, they were made by UFIP.

Interesting info in post #22: https://www.drumforum.org/threads/1923-k-zildjian-article-the-music-trades.212193/page-2#post-2537609