The Nokia Lumia 610 has already been spotted running Windows Phone Tango. Now it looks like it may be getting access to a newer version of the software (1066.0.8779.12221) which could be Windows Phone 7.8 Tango.
The current version of Windows Phone is ‘8773’ which identifies the core version. The new version ‘8779’ indicates that this could be a new version of Windows Phone.
At this point we are unaware of Microsoft’s plans on Windows Phone for older devices. Microsoft has promised Windows Phone 7.8 which some Windows Phone 8 functionality for older handsets, but we don’t know which devices will receive this update or when.
source – MyNokiaBlog