For anyone finding this now i've created a list to simply explain what each of Apple's somewhat confusing names actually mean.
iPhone Portrait iOS 8,9
- Retina HD 5.5 (iPhone 6 Plus) - 1242px x 2208px - Default-736h@3x.png
- Retina HD 4.7 (iPhone 6) - 750px x 1334px - Default-667h@2x.png
iPhone Landscape iOS 8,9
- Retina HD 5.5 (iPhone 6 Plus) - 2208px x 1242px - Default-Landscape-736h@3x.png
iPhone Portrait iOS 7-9
- 2x (iPhone 4) - 640px x 960px - Default@2x.png
- Retina 4 (iPhone 5) - 640px x 1136px - Default-568h@2x.png
iPhone Portrait iOS 5,6
- 1x (iPhone 3) - 320px x 480px - Default.png
- 2x (iPhone 4) - 640px x 960px - Default@2x.png
- Retina 4 (iPhone 5) - 640px x 1136px - Default-568h@2x.png
