Bottom. Definitely. Gots that old timey German look, which I'm sure they woulda gone with had they thought to mark the ship...
I was thinking much the same (as soon as I came across it) even though I haven't made up my mind yet.
It is odd that they didn't think of any kind of markings, not even a number registry or anything. Yet there are a number of oddities in this film. The crew don't enter the ship by any platform, but rather they have to climb in by rope ladder.
The crew take-off in a prone position (much like modern astronauts), but the controls are not easily accessible---they have to roll over sideways and reach for the control levers.
Not at all easy to do under crushing high-g acceleration (which they depict in the film).
The ship has very advanced ideas and forethought (for the time) mixed with some genuine lack of forethought.