The shield electrostatically isolates the RF amp on the right from the 3496Hz LO signals on the left. The cover must be installed with at least the 2 shield screws in order to completely eliminate the undesired signals. The shielding is adequate when the deep audio null does not shift when the RF amp is switched from Hi to Lo gain.  For this test, the RF gain pot is set to about 2/3 of max, and the loop antenna is NOT connected to the BNC input jack.  The lock alarm is on the left to be as far away from the RF amp as possible.