Sometimes manufacturers would make a base model with 15 or 17 jewels and then they would also make a model with a higher jewel count, so it made no sense to make the main plate any different from the base model. The higher jewel count version would have a cap and the jewel would be convexed (olive jewel) with no oil sink and invariably the wheel that sat in the capped jewel would be a conical pivot/arbor rather than a cylindrical pivot/arbor with an oil sink