That is a Ferrite Clamp for reducing high frequencies.

The single line out on the DMD is probably a break in the connection between the glass and the circuit board. You can continue to use the DMD in this condition, it should not cause any additional problems. In some cases you can repair the connection with conductive epoxy, although it's kind of tricky. If you're interested I can provide some links that will explain the procedure.

Thanks, so these are supposed to 'remove' high frequency hiss then?

The nature of the beast is the voltages used and the surrounding electronics will create, or "induce", noise into the data cable. This will show up on the DMD as unwanted dots, ghosting, and other issues. If the data cable was "shielded" then you would not see the Ferrites. Since they use "ribbon cables" they need the Ferrites. Where there is noise induced you can get high frequency pitch from a descret device and even noise through the sound section.

Factory.

The thing that gets me is the use of electrical tape to hold the ferrite together. Is that the way the factory installed them?

A few more years and we'll probably start getting posts "what are these black rectangles laying in the bottom of the cabinet?"

Short answer = yes. You can fiddle around with the mech to try and get rid of it if you want. It is normal with sterns, though.