I'm helping a friend research her great grandfather, who had the name 'Aron Kos Szaler' on his birth (1858) and marriage (1879) records found in JRI-Poland for the town of Krylow in Lublin gubernia. He came to the US in 1920, where he was known as both Aaron Shaler (1930 census) and Koss Shaler (New York City death record, 1941).
What sort of YIddish name is 'Kos'? I've never seen it before. From the Jewishgen given names database, the nicknames Kosiel and Kuse come up for the Hebrew name Yekusiel, so perhaps Kos is another varient. But I'd be interested in what others think.
Lak/Lock/Lok and Kalon/Kolon in Zagare/Gruzdziai/Joniskis Lithuania
Rabinowitch in Papile, Lith. and Riga, Latvia
Olitsky in Alytus/Suwalki, Lith.
Kalmonowitz in Minsk Gubernia, Belarus
Gutman and Zeligman in Czestochowa, Poland