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I could be wrong but i remember the pins where on some 265/283/327 engines.
I never used the pins, but off hand i believe the pin is the same diameter of the smaller holes if one would want to plug it/or put the pin in to hang the pump so it would pivot while assembling it.
Just a guess.
Just a guess, but that pin looks like the size of the bypass hole on the right leg of the water pump. Why anyone would want to plug that up is beyond me, but that's what it looks like. And seeing as how I'm a bit lazy, I either glue the gaskets to the pump, or just stick them in place with grease so I don't have to scrape the block when I remove the pump.