under plate

armored front

In builders” hardware, a lock front which consists of two plates: the under plate (an unfinished plate fastened to the case) and the finish plate (a plate which covers the cylinder setscrews, thus protecting them from tampering, and which is fastened to the under plate); used on mortise locks.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.