Actually, I believe the 'stacking-swivel' screw has more threads than the 'sling-swivel' screw. However, the screws can be mixed and still work (and probably often were mixed up).

It doesn't really matter, either can be made to work. Original swivel screws will often have the tip 'upset' or 'mushroomed'. It is best to use a reproduction screw and save an original from destruction.

It is necessary to reduce the diameter of some of the unthreaded shank so the screw can enter deep enough to firmly 'clamp' the sight base against the receiver.

(I probably could have left a couple of more threads on the altered screw in the picture, but, it works fine):

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