USS Ambassador
The USS Ambassador was a starship mentioned in several production sources, but never referenced on screen. 

It was first referenced in an internal list of starships, dated , where it was listed in reference to the , as an  ship with the registry of NCC-10521.  

The ship was later mentioned in the Star Trek: The Next Generation Writers' Technical Manual (3rd ed., p. 28) and in the Star Trek: The Next Generation Technical Manual (p. 75). In the latter source, the ship's registry was given as NX-10521. 

The Ambassador was referenced in the Ambassador-class article in the  and subsequent editions, as well as , where it was identified as the prototype ship with the registry NCC-10521.

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Unused production material
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