Really hard to tell but I'm leaning towards it being a 5.0, reason being that to add a boom to a 5.7 would involve too much work for the average wanna-be-sailor. A boom of the exact length would have to be sourced and it would have to have the correct goose-neck to fit the sail slot. The clew of the 5.7 has a tab that extends off the foot of the sail with grommets for adjusting the position of the main blocks, cant see that in the pic clearly, so really hard to tell. Unless the current owner measured correctly, 5.7 m converts to 18.7 ft.
http://sailboatdata.com/v…record.asp?class_id=5825
http://sailboatdata.com/v…record.asp?class_id=5826