The 649 has a shrouded or hidden hammer, note the picture with the open cylinder. This means you can still cock the revolver for single action firing.
This stainless revolver is in excellent condition. A great thing about a 357 Magnum is revolver is that 38 Special rounds can be fired from it as well. That makes target shooting cheaper and tamer, and if you want a little less firepower than the 357, you can still use the same gun.
Asking $550, no trades.