This is just an older style headset, that I THINK was made by Tange. That lockring with the three notches is exactly that, a lockring. It's threaded on to the steerer tube, not slid on like a "normal" spacer. The proper tool would most likely be extremely difficult to track down, but you should be able to use a lockring wrench that is made for a BB lockring. Also, be aware that your headset may very well have loose ball bearings rather than ones contained in a cage, thus they will fall out all over the floor when you remove the cups. Be prepared to catch them!!