I pulled it back apart and sure enough, the mark lined right up with the KAK .450 setting, right under the gas port. So I used the KAK dimple jig and dimpled one of the set screws which should make it more secure and resistant to lateral twisting of the FSB.
Make sure when you pull gas blocks you don't re-use the set screws as the knurling will not function as it did when they were new.
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