I have seen one other example of a hammer gun with a factory matted rib that was done before the usual matting was employed. It was very fine, and looked rolled, like knurling. This one here posted above looks done by hand or stamped, like stippling. The one I saw was different than this style. Here is a photo of that other one, the letter stated to “chase rib”.
You may have to open and expand the photo to see the matting.
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