The Atari 1450 XLD has the same characteristics as the Atari 1400 XL.
Like the 1400 XL, it has a built-in modem (Bell 103 compatible, 300 baud) and the speech synthesiser chip (SC-02). Contrary to the other Atari, it uses a parallel disk drive controller (a much faster arrangement) instead of the SIO interface.
Apparently The 1450 was not released because they were having problems getting the parallel disk drive controller to work properly.
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The 1400XL and 1450XLD never got to production largely because new machines like the Commodore 64 were shifting the market away from premium systems like the Apple II and more toward budget systems for the casual user. In response, Atari discontinued the 1200XL and concentrated their efforts on the low cost 600XL and 800XL.