Mopar stock steel rims....

As usual I get most of the answers in looking for here.
My question is why are certain dog dish hub cap rims on steel rims have slots (4 or 5 openings around the lip, on the inside) while others are complete steel solid rims ?

The rain I'm asking all these questions is in getting ready to put dog dish rims on my car and want to know as much as I can to make them look original, stock wheels that came with the car on said yr of it's manufacture. My case being '69.
Again I thank all the greats here with their knowledge, wisdom and know how....

Maps

P.S. This is what the car looked like when I got it, (17 in rims) going back to 15 and beautiful.

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