An Apollo 17 recovery image of the top of the CM, ap17-S72-55888 from ALSJ.
What is the black box under the parachute mortar? See the green rectangle.
It might be a part of the recovery system, a flashing beacon light or a radar transponder.
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I am not 100% confident in this answer. But I think it is the power supply for the flashing beacon light.
The flashing beacon light itself is indicated in this crop of the picture from the question.
From this diagram extracted from the News Reference Manual the beacon power supply / battery box is in the right place. But the drawing is not clear.
I am not 100 % confident in this answer. I found an Apollo 16 image S72-36328 from ALSJ:
I added green arrows to indicate three electronic boxes, a red arrow to the flashing recovery (light) beacon and two cyan arrows to recovery antennas.
Figure 1-9 from the Apollo Recovery Operational Procedures Manual showing both VHF recovery antennas and the flashing light beacon.
So the left box (of the top image) is the Sea Dye Marker and Swimmer's Umbilical, not an electronic box.
The two boxes to the right could be the flashing light power supply and the VHF recovery antennas electronic.
To build a powerful flashing light recovery beacon large condensators are needed to store the energy for a powerful bright flash. The flashing light should be visible in bright sunlight over large distances. So the bigger black box should be the flash light power supply and the smaller grey box the VHF recovery electronic.