As it turns out, the tipster who is behind the leaked photos says that the Sony D6503 Sirius is indeed the Sony Xperia Z2, making it a flagship model and a sequel to the Xperia Z1. According to the tipster, the phone will be released in April and will no longer include the printing of the Sony name on shatterproof film.
Also coming at the same time as the Xperia Z2 will be the Sony DGP521, which is a new flagship tablet allegedly being released by the Japanese manufacturer.
The Sony D6503 Sirius and its two predecessors
1. The Sony D6503 Sirius in black and white
2. Sony D6503 Sirius side by side with the Sony Xperia Z1 and the Sony Xperia Z
Thanks, Anonymous Tipster!
source: Digi-Wo (translated) via XperiaGuide