Koerstan is nothing more than the Anglicized version of the German O-umlaut. I can't put that into this reply but think Boker (pronounced Becker) knives. See this link for explanation.

http://en.wikipedia.org/wiki/Umlaut

Koerstan gives you the phonetic spelling and Kerstan is likely further shortening. I bet it comes from Korstan (o-umlaut or with a pair of dots above). Anytime you see a Germanic name in America that includes "oe/oer" think phonetic spelling for o-umlaut.

Shoot straight,

Bird