A "plank owner" is an individual who was a member of the crew of a ship when that ship was placed in commission. In earlier years, this applied to a first commissioning; since then, it has often been applied to one who was part of a recommissioning crew as well. "Plank owner" is not an official Navy term, and has consequently been variously defined by different Navy units.

Plank-owner certificates are procured by and issued to crew members of the ship being commissioned; they are not officially issued by the Navy. Some ships' crews design their own, while others purchase them from commercial sources. Perhaps the best-known of these are the ones sold by the United States Naval Institute. These color certificates can be obtained as blanks; if the purchaser wishes to provide the necessary information, they can be filled in for an additional charge.

compliments US NAVY
Source: http://www.usssaufleydd465.com/plank.htm