Better Pack Electric Models with the two prong twist type plug. Also used on machine prior to 1976 with heaters wired directly into the machine.
Note: If your tape is not sticking the heater may not be the problem. Moistening brushes need to be cleaned every week or two with dish-washing detergent. Avoid hand soaps as many contain oils. The bristles on the brushes will scratch into the glues to reactivate the hardened dried glues and help the water penetrate into the glue. Worn brushes will cause the water to bead up on the tape and give dry spot with uneven moistening. Brushes should be replaced every 3-5 years depending upon usage.