The 58 bird has 2 vents as noted in previous posts and non ac and AC bird are the same. The vents very often rust up slightly making the knobs very hard to pull on the 58's I hav eben lucky as mine have worked as long as I have owned the birds. Very easy to break the black knobs if trying to force the vent open. 58's the vent is opened by pulling the knob straight twords the cabin. Not easy as there is a couple pivot points that are not straight.
There is no other vent system that I am aware of on 58 birds.
Sun Prairie, Wi.