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I just went to MSD's web site and I don't see the vent on the marine distributor caps like they used to be and the description on the marine distributor states the vent is now part of the housing.
I did see their race caps had what looked like a vent on the top of the cap.
You might call Holley (MSD) to see, what will work for your needs, if they still sell the old style marine caps or if the vent is still in the cap but not shown on the photo on their web site.
Good Luck.
The MSD shaft is 0.500 and a stock GM distributor shaft is 0.491. Summit has the ACDelco Distributor Gears 10456413 (melonized) this has the 0.491 internal dia.
I thought the would have made the shaft really thicker to have any reasonable advantage.
But this is just to sell their own special priced spare parts.