Presumably there is evidence for both explanations. However, I lean towards Mr Mangles', because the Dutch way of writing an 8 is to begin at the waist, not at the top, and to end up with the shape of a stencilled "S". I would expect this to be a standard usage in pre-independence New York, originally colonised by the Dutch. Some form of cancellation would be needed to distinguish the sign from succeeding figures, hence the bar.
Incidentally, the daler is still visible in the name for a two-and-a- half guilder coin, the Rijksdaalder, popularly known as the Rix.
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