The number 5 stamp designates the impeller used inside. There's other factors that determine stall speed so if there's no stickers or markings, the only way to know what you have for stall is stick it in a car, or send it in to be refreshed. I would bet it's from Precision of New Hampton, but others may use blue paint also. If it's a stock diameter, then it could in theory be anywhere from 1000 to 2800 stall speed.
How did you come to own this converter?