That 10-lbs of coal per hour must scare away a lot of potential buyers. 10 x 24 is 240-lbs of coal per day which can get pretty expensive.
If they aren't scared by the 1.0 gallon per hour firing rate stamped on their oil furnace, I would hope 10 lbs/hr of coal isn't a shock.
Why is the firing rate so different from the 520 when they use the same stoker?
The AF150 heat exchanger can't handle the full output of the stoker.
As for the Van Wert, the feed rate is only changed by changing the pulleys on the stoker; it is normally "factory set".