thanks Burdar...I just ordered a new locking nut...I went back to the road I lost the pulley on, but who knows how long it stayed on after the nut had already fallen off.
You've got threads on the end of the pump shaft. That tells you that it's a "bolt-on" style pulley.
Put the pulley back on and check to see if it's bent. If it's straight, it should be fine. The nut will keep it on. The nut should be a "locking" style nut.