After a bit of doodling, to see how someone might have gone about producing the swastika carving it seems to me they most likely first produced a cross consisting of 9 cup marks, then drew a line, for whatever reason, first to the left and then to the right of each cup mark in succession, arriving at the shape as a result of the rules they were following, rather than deliberately trying to make a representation of anything.
The alternative - to draw a boomerang or throwing stick or whatever - and then add cupmarks both inside it and outside seems a bit odd.
