Who is the skeleton on the final bulwark shield?

ive noticed that the skeleton on the last unlockable bulwark shield has the same proportions sizevise of the primaris marine wielding it. The skull, ribcage, humerus, ulna and radius and femur bones that are intact fitt if you compare them to your caracter if you ignore the mass the armor adds on the model. Are the bones made of artificial material shaped into bones or is there a already dead primaris chad marine straped to our shield here to tell us the time incase our visors fail to tell us?