Yup, it's a "flap" covering, the only thing, as bbbobbb said, is aligning the nubs on the plug with the 6412. I just wanted to say that the series III "even so" was harder to push in, and gave a very definite "click" when the optical cable found it's home. The good thing about that is earlier boxes that I had didn't firmly grab the optical connector, so I look at it as a slight improvement in quality. Of course the poor, quirky performance of the PVR itself makes me wonder why Motorola seems so technically challenged, when Tivo had the PVR interface thing down about 5 years ago...
UPDATE 10/27 - Yesterday I exchanged the 6412-III for Yet Another One... and though it's been less than 24 hours, WOW. The old one would freeze picture and sound "every now and then" for 8-15 seconds, then resume, nothing lost. Then again, it would lock up about once a week - really lock up. Recordings from the night before, not made, and totally ignorning IR commands, or sometimes letting me display recorded programs - but would show a red status bar, no playback possible. I'd pull the plug for a minute or so, and it would restart, recorded programs intact (older ones, that is).
There was discussion here before about the 6412-III doing a better job on analog channels, and I said that after a few days I had decided Moto added "smear-o-vision" to make the picture look "better." The new box has great analog picture presentation, and all pictures are better in terms of brightness and contrast (still using component, until I can afford a new TV, and having retired, that might be awhile). There is now very little need to "tweak" the TV brightness/contrast when switching from a non-HD channel to HD, PQ is just plain good on all channels.
All this to say that if you have a cable box that just isn't right (those mini-freezes were really maddening) - get another one. I asked the guy at the counter at Comcast if the 6412-III was the latest, he said it was, and there was nothing new or different available in a HD-equipped PVR. I left not feeling very confident, thinking this one might be a "freeze and lock-up special" as well - but it works VERY well. The day may well come where the box Comcast gives you the first time works correctly!
The box is now delivered with default "TV Type" as 16:9 which is the first time I've encountered that. I have one of them old-fashioned Panasonic Tau CRT's so yes, while I see the whole world has gone 16:9 when I look at Best Buy, those who have "legacy" HD monitors (4:3 tube but will do HD with black bars top and bottom) need to change that setting right away!