Those are official abbreviations. Kosciusko is Ko, Madison is M. http://www.in.gov/dnr/historic/files/countysite_designations.pdf
Hmmm... The list appears to be about the county designations used in archaeological site numbers. I'm not sure that makes them the official abbreviations, nor would a typical Hoosier know about them. The page is linked from here: http://www.in.gov/dnr/historic/3668.htm I found a completely different set of abbreviations used in County Formation Maps, on various sites, such as this one: http://www.mapofus.org/indiana/ Not sure if either set are official, but the second set at least seems more sensible.
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Thanks. I hadn't heard that before. Spec somewhere? -Matt