It sounds to me like it might be one of the older models that Yamaha had before they changed their model numbering system.
The old 600 series models are usually equivalent to the current 700 series ( I think...). If this is true, then it should be a flute that is entirely sterling silver (head, body, foot and keys). From what little I could glean from a quick google search, it seems to be a model that was sold in Great Britain. The ones that I saw online generally had closed holes, french arms, offset g, and sometimes a split e mechanism.