The image above is the derailleur that will go on the Trek 520.
I just purchased it on eBay for $92, including the shipping - yes too much.
I have a couple sets of the Suntour sealed bearing pulleys. I will use one.
If the Cyclone front derailleur I have will not handle a triple, I have a triple Mountech that will work.
To clarify - I will have a Trek 500 and Trek 520, both grey, both 24" with relatively matching components.
(I know, only I would think that is cool.
I was dropped on my head several times as a child)
I will need to check serial numbers and see how close they are in age.