One of the things he started doing is to make them adjustable. The other interesting thing was to put wheels on them. The wheels are not so much meant for rocking back and forth, just to make the chair a little bit easier to move around.
To adjust the chair, a dowel that goes across the chair has to be removed
and inserted into a different hole. A knob on one end of the dowel is unscrewed
from he dowel to allow it to be removed.
This does make it a little inconvenient to change the adjustment of the chair, and so
the chairs don't get adjusted all that much.
Here's another one of my dad's chairs, this one in oak. Same idea
This one comes with a footstool, also height adjustable, and with
wheels. Unfortunately, the way one usually uses the footstool, the wheels
are only good for moving it side to side and not towards or away from the chair.