Unique Titleist Prototype/ Tour Issue 680/670/660 Iron

One of the most unique set of Titleist blades that I've ever seen. They seem to have been made around the same time as the 680/670/660 series, but they have a unique reverse curve of the muscle back and sole shaping. The hosels have also been stamped, but we haven't been able to figure out what they exactly stand for. The set is composed of 2,4,6-pw (7 pieces). True temper NDI Pro stiff flex and original Titleist cord grips. Loft and lie seem to be about standard. They look to have barely been played, if at all. Not really sure how to price these given the rarity. Open to offers