Originally Posted by
EuroFlash
EDIT: Finnair website says this, this caused the confusion:
Charter flights include all Finnair flights that are rented out to travel agencies for package deals. All charter flights are numbered AY1000–AY1999. The travel agency is responsible for the sale of the package deals so buying flights through Finnair is not possible.
AY+ earnings page indicates that the charter flights were AY1000–1999 until 28 October 2017 and AY2001–2500 since 29 October 2017.