The dryer design contributes to efficiency issues. Due to the perforated drum back, heated incoming air can pass behind the drum and be pulled right out the exhaust without traveling through the damp items. By the late 60s' model which you currently own, the bulkhead or drum back was solid with air entering and exiting the drum through openings with no connection between them, forcing the air stream through the drum. If you take the dryer apart and put felt baffles behind the drum to force the incoming air to travel through the drum, it will improve the efficiency.
One place this style dryer excells is in situations where you have a long vent. According to John Lefever, the dryer pulls enough heated air behind the drum to keep the vent pipe from developing condensation which causes lint to stick to the pipe which is how vent clogging starts.