The 5.5-inch iPhone -- possibly called the iPhone 6 Plus -- should ship alongside the 4.7-inch iPhone 6 on September 19, says French site iGen. It's indicated however that quantities of the 5.5-inch model will be limited. Past reports have suggested that the phone has experienced production problems, and indeed the device has been seen in far fewer leaks than its 4.7-inch sibling.
A September 19 launch date may be likely for at least one new iPhone model. Apple has a press event scheduled for tomorrow, and tends to ship iPhones two Fridays after an announcement. Conceivably, though, the 5.5-inch unit could be pushed to a later date to build up inventories.
The site claims that the iPhone 6 is codenamed the N61, while the 5.5-inch iPhone is the N56.
