ISO has assigned two new country codes for Serbia and Montenegro.
Serbia : RS,SRB,688 (FIPS : RB)
Montenegro : ME,MNE,499 (FIPS :MJ)
Link : ISO Press Release
The country code ‘CS’ previously used for Serbia and Montenegro is now obsolete and the geonames database has been updated with the new country codes. For most records we could use the administration code 1 for the update, for 255 remaining records we have used the country code of the closest records with either ME or RS.
The geonames database includes now14’056 records for Montenegro and 29’351 records for Serbia.
GeoNames Blog 
As part of an audit I am just now discovering the history of these country codes, and I truly cannot understand why in the world you would have chosen CS, formerly used for Czechoslovakia, as the code for Serbia and Montenegro. I guess somebody wanted to create complete confusion.
CZ – Czechoslovakia – not CS
CZ is the ISO code for the Czech Republic. Czechoslovakia diasolved in 1993 into Czech Republic and Slovakia. It had the ISO country code CS.
I fully agree it was a surprising choice indeed.