What is the metal plating on the back of the A-6 for?
It is a speed brake, a plate hinged at the forward edge which can be hydraulically extended to increase aerodynamic drag. It had been used on early models, and the speed brakes were mostly replaced by firmly riveted plates later, because flight test revealed handling problems. The wake of the speed brake disturbed the flow around the horizontal tail especially during approach and landing. They were replaced by wing tip brakes, and from the 304th aircraft on were completely omitted.

See here for further details.

Below is a picture of an A-6 with the wing tip speed brakes (circled red) extended:


(Source: navy.mil)