Dan, I see the filled holes now. I read that in the auction description but couldn't see them on my laptop. I would say that it is indeed a factory fill job considering the overlay is over the filled holes. I've seen one other Dobro made by Semie with this overlay foil treatment. It makes for a stunning headstock! Semie was known for altering necks and headstocks to suit his purpose at that time. I can show you headstocks where he added wood to the end of it to make it longer. Years later a hairline crack is evident in the clearcoat.
I do not have a clue about the DC prefix. It could merely have indicated that these groups of guitars were made at a different location or time period. For example after 1972 when Mosrite started up again. We see the same serial number changes in the other Celebrity models.
Dennis, thanks for the YouTube link. Now I can say I've seen a 12 string Dobro! I should not have spoken so soon, eh.

I really like the song in that video!