The original pilot for Lost in Space, titled "No Place to Hide", used a different ship for the Robinson family's journey. It was called the Gemini 12 and and, apparently, had only one deck for living space. But the Lost in Space Wikia, on the Gemini 12 article, states:
The Gemini 12 was the name of the Robinson's vessel in the original unaired pilot “No Place To Hide.” It was similar to the Jupiter 2 on the exterior, except for a few slight differences in the ships’ profiles. Footage of the Gemini 12 would often be used as exterior footage of the Jupiter 2 creating some inconsistencies, although it is not usually noticeable. The interior however, was filled with differences. A major one being the Gemini 12 had no lower deck. Also, several control panels were different.
This talks about the interior differences and it references the exterior ones, but it does not give us any idea of what those exterior differences are. Apparently the two are similar enough most people won't notice the differences between them.
Is there a listing anywhere of the exterior differences between the Gemini 12 and the Jupiter 2? Are there any clear diagrams or plans (other than poor quality images of blueprints) of what the Gemini 12 looked like internally or externally?