Going to No. 00 aswell, I think it means Izayoi. Let's take a example from Megman series. First of the maverick series robots is obviously DWN-∞, who is commonly called Zero. Now compare his design http://images2.wikia.nocookie.net/__cb20100601214326/megaman/images/thumb/4/45/X1_zero.jpg/371px-X1_zero.jpg to the next DWN-∞ sries robot, Megaman Zero http://images2.wikia.nocookie.net/__cb20110715205749/megaman/images/thumb/7/71/RMZCWp144a.jpg/640px-RMZCWp144a.jpg and you notice that they look really different. So where Zero is the first one, he's more of a prototype as the general design of them changed after him.
Now if you compare Izayoi's design to the Murakumo Unit's design, you notice that they look quite different. So in my logic, No. 00 means a prototype a.k.a. Izayoi models (they created multiple Izayois). Saya would be the first true Murakumo Unit a.k.a. Zerp