The front-most roller, which I haven't mentioned yet, is mounted on two sliding mechanisms whcih are mounted, in turn, on two screws that I can turn with black knobs on the front of the machine. That's my belt adjustment.
The wing nuts are on bolts that hold one side of tension spring on each side of a large black roller which is the roller which initially presses the apron against the drum as the prints travel into the dryer. The springs only control the amount of pressure exerted by the roller onto the drum itself. There's no space between the roller, apron or drum at this point, so it's not an apron tension adjustment.
Age? I really wish I knew. If the date is encoded in the serial number in some obvious fashion, maybe it's from 1962 or 1972 (more believeable).
Since details about the dryer are cropping up in this discussion, I grabbed some snapshots of it and put them at http://bellavestudio.com/tmp/printdryer/.
There's an overall shot, a shot of the nameplate (it's a Printz Automatic Print Dryer, model Jet, no. 2271.), a shot of the aforementioned knob/screw/roller for apron adjustment, and an attempt at a shot with the side plate removed to show the wing nut/screw assembly that controls the tension on the entry squeege roller assembly (*shrug*) or whatever it's called.
Bear in mind I've still got the apron removed. I've still got to go over the drum with bonami before I replace the apron.