They'll have different model numbers, but as with most manufacturers they just replace models in the same range with similarly named models.
The current model is the SS-650KM3 and the primary difference is they've added a couple more years to the warranty.
They are all a bit different inside as they tweak bits and pieces over the years but they are all pretty solid. The only concern I'd have is that the KM3 range has been out for around 18months or so, you'd have assumed most of the older models would have been sold off/left the supply chain by now.