Originally Posted by
zzyzx_xyzzy
It sounds like the OP has one of those headsets whose keyed washer has saw-tooth serrations that mate with corresponding sawtooth serrations on the top race, like this:
Yup, mine is almost
exactly like that, though mine has a second, non-keyed and non-threaded washer between the locknut and the keyed washer. The section with the flats on the locknut is considerably shorter too. But the top cup and the keyed nut in your picture look like they may be the exact same parts I have.