Bluetooth is available as a build-to-order option, which is installed by Apple at the time of purchase as a fully integrated module. Accessing the Bluetooth capabilities without purchasing the integrated module will require a third-party dongle that may not support all present and future features supported by the internal module.
When the Bluetooth option is included on the factory order, the Bluetooth antenna is internal to the enclosure.
Very strange. Why didn't they make it standard?