Good for you!!
There may not be a tension "cover". I'm not sure about other models, but the 66 doesn't have a cover. Nor does it have numbers indicating what the tension setting is.
I recently purchased an old abused treadle myself. The cabinet top is in critical condition but the drawers are in fair to good condition (as are the drawer frames). I've given husband carte blanche to restore the cabinet while I restore the machine. He has already decided to keep the drawers and completely replace the top. And he decided that instead of folding the machine down into the cabinet, he's going to recess the machine to the base then fit it with a coffin top. As these cabinets are not really worth that much even refinished or in prime condition, we're not worried about restoring it to match it's original look. We're after functionality and practicality.