The search continues! For what it's worth, there seem to be a few different varieties of "black barred silver dollar." The species most often associated with this common name is Myleus schomburgkii. This - I believe - is an adult of that species:
There is another "barred silver dollar" that seems to appear though, and these have a red anal fin and a narrower black bar than the above species, looking more like this:
I'm not 100% sure that these are different species. The top fish is obviously a mature adult and the second is a younger fish. But from what I've found, they are at least different variants of the same species, and possibly different species altogether.
In any case, I'll take either one if somebody can find them.