Reading all of the above, I think that the name Kos is from Kehos/Kohos/Kehat/Kohat, from the name of one of the son's of Levi. All very interesting.
I looked on SteveMorse to see if there are any New York City death records for the given name Kos or Koss, and there are about 10 of them, all from Jewish families. So, there were a few men walking around NYC with this name.
Thanks for the help.
My friend spent 5 years off and on trying to find her great grandfather by searching exactly for 'Aron Kos Shaler' and only with that spelling. Couldn't find anything. I found him in about 15 minutes by searching census records for anyone with the last name 'sounds like Shaler' in NYC, and found Aaron Shaler living in Brooklyn, age 70 years. And using Steve Morse, found the death record for Koss Shaler. Sure, I got plenty of Schillers and Schollers, and the like, but it was safe to ignore them.