At about the same time we got the Dungeon Masters Guide, we also got the Dungeon Masters Screen, in fact during the year of 1979 we got two editions of the Screen, the first one with just an orange cover with the title but no art and soon after another with an Erol Otus knight and dragon drawing on the cover. 

As usual, the DM screen is full of useful tables for the DM to use while guiding the session as well as being a useful way to keep players from snooping on his die rolls or to have glimpses of the adventure coming up.