. . . necessarily be originally fitted, Jim, for two reasons. First, I've seen several American Seamasters with the rounded crown, including a ref 2627 I've just discovered locally. Second, and more to the point, I was able to confirm the crown on my '56 ref 2849 (US ref KX6275) . . .
. . . is the one originally fitted, so the general rule regarding clover-leafs doesn't always apply.
If you need to replace the existing crown for either mechanical reasons or to maintain water-tightness, then a clover-leaf would be the right choice.