There are 3 locking standards but only 2 are common. Sheldon is your man.
http://www.sheldonbrown.com/gloss_l.html#lockring
FWIW I had exactly the same problem a couple of years ago. I couldn't work out why the locking I'd been given with the hub wouldn't go on properly until I found out that there are 2 sizes and I'd been given the wrong one.
@yoav started
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There are 3 locking standards but only 2 are common. Sheldon is your man.
http://www.sheldonbrown.com/gloss_l.html#lockring
FWIW I had exactly the same problem a couple of years ago. I couldn't work out why the locking I'd been given with the hub wouldn't go on properly until I found out that there are 2 sizes and I'd been given the wrong one.