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Detail of the interface between the re-entry control section (right) and the cabin (left).

The groove running along the top of the spacecraft is the main parachute bridle trough, which holds the main chute lines until the spacecraft transitions from a single point to a two point suspension system.

I'm not certain what the white circle just this side of the bridle trough is. I found a diagram which indicates that there's a C-band antenna in this general location, and it certainly looks like the C-band antenna on a Mercury spacecraft, but I'm not certain.