It is the assembly that drives the seed box.

I had the same problem (with the "dogs", when I first got my 1200, but maybe they were just weak because I havent had any more trouble.  Also, their quality control sucks on chains, too.  Hardly any of my chains had keepers on the master link, and I found this out the hard way!!  I ended up putting a lot of AMERICAN chain on there, and have not had any more problems. 
They appearantly roll this stuff out without checking it!!  Heaven forbid they check out their product !! The chains coming off was enough to get me fired up.  I mean, on the first one or two I was ok, but when it kept happening, I let my dealer know about it.

If I would have purchased this planter used, then I would have expected some trouble, because it just goes with the territory.  Even when new, you will get a few gremlins---but for it to keep on happening...that is just wrong.