I'm not sure if anyone knows the exact timeline placement of the photos or not but that photo is what I've always assumed to be one of at least two pre-production photos taken that show the blaster how it came from Bapty, before the SW prop guys got their hands on it and added the Paint, Disk, Grills, Barrel Greeblie and Weathering.
If the SW guys indeed added those details rather than being revisions made by Bapty themselves after the pre-pro photos, the mystery disk seems more likely to be just masked off into the black Paint that the SW guys or whoever added. Also aside from the few custom machined and fabricated items here and there(i.e. Vader), in ANH the prop guys didn't show really any evidence of having access to the kind of machinery or experience needed to Mill in the Mystery disk as perfect as it looks. So to me the disk seems likely to be just masked into the paint whenever the paint was added. And like lonepigeon has said, the Imperial greeblie may somehow be involved in the 'mystery' of the disk too.
I would always love to find out more about the true order of events that resulted in this beautiful blaster!