These are robot arms with double missle launchers, fully compatible with Warhammer 40k Dreadnought models. The models includes both the right and left arm.
When you recieve the arms they are connected with small sprues. Cut or clip off these with a model knife or a pair of pliers. The arms can easily be magnetized with a 5 mm diameter wide and 1mm thick neodym magnet.
White strong & flexible is the simplest material, all details will be fully visibel. The material has slightly rough surface. I recomend brushing of any excess powder with a thick brush and use a primer to create a good adhesion before you paint them.
Frosted detail is the optimal material and will end up with a smooth surface and crisp details. There might be an oily coating on the model after printing. Please wash the model in water with some soap or mild detergent before priming.
You can add an image to the left arm and 1-3 letters/numbers to the right arm, for example 'IX'. This could be your chapter insignia and id. You can use any black and white square image, or the examples below. Have you not tried Shapeways customizable functions I reccomend you try it out. You get an instant 3D preview.
See also my Dreadnought autocannon arms