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Use $y/b$ horizontally and $z/c$ vertically, and label each molecular center by its fractional height $x/a$ along the viewing direction. The four centers in one periodic cell are $(0,0)$ with height $0$ and orientation A, $(1/2,0)$ with height $1/2$ and orientation A, $(0,1/2)$ with height $1/2$ and orientation B, and $(1/2,1/2)$ with height $0$ and orientation B. Boundary images complete the projected rectangle. The A and B bonds have opposite slopes; the atoms in each bond are indistinguishable.
\Image[/past-exam-of-the-materials-science-course-of-the-university-of-cambridge/2016/paper-1-iodine-projection.png]
{title=Original molecular-center construction and symmetry traces for the iodine cell viewed along $[100]$; bond inclination is schematic, heights and bond length are specified}
The height labels matter: a symmetry of the unlabelled projection need not be a translation or reflection symmetry of the three-dimensional crystal. The drawing uses the given $0.267\,\mathrm{nm}$ molecular bond length and an illustrative inclination, since no exact inclination angle is specified.
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