v_27: New methods in Key class, updated behaviour of activated machines, Payment Form methods
The major change is the introduction of the latest model used in Key.activate and Key.get_key. You can read more about it here. Below is an excerpt from the Web API Docs about ModelVersion 3:
This model has a few changes. First, we have added a Reseller object to the LicenseKey. Secondly, we have made sure that all node-locked and floating licenses are returned (unless field masking is used). We added FloatingExpires to ActivatedMachines so that you can check when the activation was set to expire (note, this value can be overridden when calling this method with a different FloatingTimeInterval).