The 610 Duo was the initial model of the charger, it's the older model. The 615 is the current model, DuoIII. 610DuoII was inbetween these two. A new model SuperDuo3 is expected this fall.

Check here toward the bottom - all the Hyperion chargers are here with info about each - look at the manuals, the features and capabilities are summarized on the 1st page in each.

http://media.hyperion.hk/dn/eos/