I'm with SC, the BM would likely have started with diluted citric acid on silver coins, probably
applied with a cotton swab (dealers often just soak them outright in lemon juice, but the BM would be more careful). They would have mechanically picked off enough encrustation to reveal it was silver first.
Bronze coins would have been treated quite differently by the conservationists.
It's common that some deposits from adjacent coins or soil mineralization would remain after the obvious non-invasive methods. In this case, the remaining deposits do not take away from the coin and I too would have left them alone.
A dw soak with the water changed every day for a week or so would completely neutralize the citric solution.