I supply the swivels polished in the white. I believe this was the way they were put in original guncases, as the ones I've examined have showed no evidence of being blued. "Polished Bright" was once a very popular finish. Looked great in the shop, but not so long in the field.

The heat treat I do shouldn't harm the sling eyes, it is a spring temper which is easily cut with a file. There will be normal metal on metal wear though, as the sling hooks are a fairly loose fit in the eyes.