The SDF model is the non-neodimyum model of this driver.
Very similar specs to the more expensive version, sound is almost identical. Your not going to hear the difference. Only issue is the chamfering that needs to be done on the back side of the baffle to let the drive breathe and prevent rear sound waves from reflecting. Great driver and can be pushed pretty hard. Excellent mid for the price. Compares nicely to Zaph's