Something missing...?

The stills you see printed in publications are almost invariably production
stills rather than frame enlargements from the film negative. They
frequently don't replicate the angle or the framing of the shot being filmed
at the time. It is often the case that all sorts of embarassments which are
framed out of the shot creep into a production still.

It wouldn't surprise me if, with a complex model like Moonbase, it was
built with removable sections. This would make it easier to work with and to
gain access to the different parts of the model. The missing section
probably either wasn't in the frame of the shot being filmed at the time or
the still was taken between takes which would also account for the model
craft possibly not hanging in it's final position.

Sorry if this explanation sounds dull but it's very plausible.

Regards
John