This is a robot arm with a missle launcher, fully compatible with Warhammer 40k Dreadnought models. The models includes one left arm.
Photos of the same base model of the missle launchers can be found here
The arm can easily be magnetized with a 5 mm diameter wide and 1mm thick neodym magnet.
White versatile 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.