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I was going to say that the addition of the ring may have been primarily for sound; aesthetic.
What difference in diameter and blade pitch would be the other questions.
And, how is the "fit" different? The open blade type wants to be "in" the shrouding around half the width the blade or more. Plus a clearance of 1/2" or so from blade end to shroud.
These small differences can add up in cooling efficiency. An oversized radiator -- by comparison -- is needed if airflow is compromised.
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