a) Serpent from The Eternals
Another strange source of reference for the alien derelict.
It seems that Giger must have been looking through Jack Kirby's The Eternals when he was coming up with further ideas, and by the looks of it he had been a fan of his work for a few years already at the time.
From #9 of the comic books, the flying serpent guarding the city of Olympia in this comic book double page spread appears to have three mysterious circles on its side.
|Flying serpent over Olympia. Double paged spread from #9 of The Eternals
|Alien derelict ship (source: Giger's Alien, p26)
b) Hammerhead details
Also taking note of the five holes on the hammer head tip of the derelict ship, at the tail of the serpent in the comic book illustration, there is a series of six wiry ribs that might have set off the idea for that.
So this would have been incorporated into the design of the derelict in the later stages of the conception of the design.
However it turns out that Giger would have known that the flying reptile referenced his work 147 where in that same position in the drawing there would have been a row of oval holes near enough like the row of holes on the hammer head tip on the derelict in Alien.