Are your screens simply the kind with a weighted bar at the botton that you pull up and down with strings or chains?

Perhaps it makes more sense if one reads: "..fastening to fabric roller tubes and ballast" (unless of course, the ballast (rod) at the bottom of the screen is pushed into a tube).

I don't think there's an elliptical construction here: I reckon you've got ballast at one end and a fabric roller tube at the other. Where the plural "tubes" arises is indeed a question. No idea what actual German terms to use, to be frank...