You can get the rubber grommets at most parts stores as well as the twist on type oil caps. The "indentations" are for a twist on type oil cap or breather. You can also buy rubber plugs to fit the holes in case you wanted to block one off if you use a twist on type breather instead of an oil cap.

Mike
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1958 Chevy 3200 Fleetside
1958 GMC 100 Wideside
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