I've just done a search on Jewishgen, looking for entries in the Lublin Gubernia for the given name Kos. And I got 62 hits. I found one from 1847, where its easy to see the father's name Kos Mandel written in both Polish and Hebrew. Perhaps this will shed more light on this given name. I take it that in birth records, a father's religious Hebrew name would be used, and not his every day Yiddish name.
