This post explains how to determine model year and casting date of heads.
1.) Year Cast. This represents the actual year the head was cast.
2.) Shows the first Model year this part number was available for. The 99-05 castings all have -99.
3.) Front, Indicates the which head it is. The other option is Rear as shown on some of the other pictures.
In this picture you can see the year of casting is 2000 and could have come off a late 2000 OR early 2001 model.
This was cast in 04 and takes the later valve seals and has guides for 7mm valve stems. This means it is for a model year 2005 bike.
Note this picture shows a new part year designation of -06. This means it is from a 2006 or 2007 model year and has the new style ports. Year of casting is 2005.