Apple's long-rumored AR/VR headset will enter mass production in October and launch by December, according to investment firm Morgan Stanley. Apple is still expected to unveil the headset at WWDC next week, and provide developers with tools to create apps for the device, which is expected to have its own App Store. Apple headset concept by designer Marcus Kane "While we expect Apple's AR/VR headset to be unveiled next week, our supply chain checks suggest mass production won't start until October '23, with general availability most likely ahead of the December holidays," said Erik Woodring, an Apple analyst at Morgan Stanley, in a research note obtained by MacRumors. Apple's supply chain is preparing to assemble only 300,000 to 500,000 headsets…