Just checked the cover of my HOPB DVD and I feel it's safe to say this isn't public domain. It has a 1963 copyright for the Rainbow Film Holdings, LLC, which is a film library owned by VOOM. To back this up a little more there is a small Monsters HD logo on the front of the box.

Overall, the DVD is lovingly produced as I and many others have come to expect from Dark Sky Films. Informative, if sometimes rambling, audio commentaries for HOPB and The Curse of the Living Corpse, and a nice interview with Del Tenney, most of this information is also covered in the audio commentaries with more detail, but it's still nice.