Magnetic Anomaly Detector, I believe.
Designed to detect magnetic anomalies (metal) in the water from the air. I.e., submarines.
Which is a subtle way to introduce the flap over high-flying fast movers vs. low & slow for ASW work.
They almost need to pull a bunch of P-3's and P-2V's off the scrap heap to "augment" this new force of 500-kt highflyers.