Wikipedia
In printing, a key plate is the plate which prints the detail in an image.
When printing color images by combining multiple colors of inks, the colored inks usually do not contain much image detail. The key plate, which is usually impressed using black ink, provides the lines and/or contrast of the image. However, in two-color images where neither color was black, the key plate might have been printed in the darker of the two colors. Key plate is synonymous with keystone in the same context.
Usage examples of "key plate".
The first door she'd tried had been locked, a carving of a helmet on the key plate.
The key plate of the door frame was glowing in welcome as she approached her assigned room.
Placing his hand over the bronze key plate, he examined the intricacies of its mechanism.