Putting a shackle on the chain plate over, as it were, the turnbuckle fork is not an uncommon way to make an auxiallary attachment. Because 1/2 does not equal 5/8, there are potential problems.
If the pin and hole are not reasonably matched, you get unfair wear on both and an unfair bending strain on the pin. This, combined with the fact that the smaller pin is weaker, may put the rig below acceptable specifications. In practice, much in cruising yacht rigging is significantly over-engineered and a modest decay in strength might be tolorated but I'd certainly wonder.
I'd especially wonder since it's just a preventer and there are so many excellent ways to make an attachment without erroding the integrity of the rig as it is. For example, you might (quite neatly I might add) selvagee a shackle right around the chainplate below the turnbuckle fork. Along that line, leave out the shackle always on the selvagee, just attaching the shackle or 'biner on the preventer directly to one turn of the selvagee. That way there's nothing clattery when that side's to weather.
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